Make flake8 happy

src/ZODB/DemoStorage.py:388:80: E501 line too long (89 > 79 characters)
src/ZODB/DemoStorage.py:391:80: E501 line too long (87 > 79 characters)
src/ZODB/DemoStorage.py:393:80: E501 line too long (83 > 79 characters)
src/ZODB/DemoStorage.py:395:80: E501 line too long (87 > 79 characters)
src/ZODB/DemoStorage.py:397:80: E501 line too long (100 > 79 characters)
src/ZODB/DemoStorage.py:398:80: E501 line too long (86 > 79 characters)
src/ZODB/DemoStorage.py:408:5: E303 too many blank lines (2)
src/ZODB/tests/testDemoStorage.py:397:9: F401 'ZODB.FileStorage.FileStorage' imported but unused
src/ZODB/tests/testDemoStorage.py:399:80: E501 line too long (101 > 79 characters)

loadAt -> loadBeforeEx since it was renamed so on
https://github.com/zopefoundation/ZODB/pull/323/commits/805bf36f .

* y/loadAt.8: (35 commits)
  Make lint happy
  Let the year float.
  Configuring for pure-python
  Specify a PyPy2 version.
  Lint the code.
  Configuring for pure-python
  Fix TypeError for fsoids (#351)
  Fix deprecation warnings occurring on Python 3.10.
  fix more PY3 incompatibilities in `fsstats`
  fix Python 3 incompatibility for `fsstats`
  add `fsdump/fsstats` test
  fsdump/fsstats improvements
  Undeprecate loadBefore
  fixup! changes: Add draft entry for loadAt/loadBeforeEx/DemoStorage fix
  changes: Add draft entry for loadAt/loadBeforeEx/DemoStorage fix
  fixup! Handle NotImplementedError raised by loadBefore/loadBeforeEx as "interface not provided"
  *: Don't emit warnings on loadBefore
  Handle NotImplementedError raised by loadBefore/loadBeforeEx as "interface not provided"
  loadAt -> loadBeforeEx
  - add coverage combine step
  ...

Sorry, but some of flake8 complaints are really stupid...

Resolve many conflicts after https://github.com/zopefoundation/ZODB/pull/357.

* master:
  Let the year float.
  Configuring for pure-python
  Specify a PyPy2 version.
  Lint the code.
  Configuring for pure-python

* Config with pure python
* Lint the code.
* Add support for Python 3.9 and 3.10.

to resolve trivial conflict on CHANGES.rst

* origin/master: (22 commits)
  Fix TypeError for fsoids (#351)
  Fix deprecation warnings occurring on Python 3.10.
  fix more PY3 incompatibilities in `fsstats`
  fix Python 3 incompatibility for `fsstats`
  add `fsdump/fsstats` test
  fsdump/fsstats improvements
  - add coverage combine step
  - first cut moving tests from Travis CI to GitHub Actions
  - ignore virtualenv artifacts [ci skip]
  tests: Run race-related tests with high frequency of switches between threads
  tests: Add test for load vs external invalidation race
  tests: Add test for open vs invalidation race
  fixup! doc/requirements: Require pygments < 2.6 on py2
  doc/requirements: Require pygments < 2.6 on py2
  fixup! buildout: Fix Sphinx install on Python2
  buildout: Fix Sphinx install on Python2
  Update README.rst
  Security fix documentation dependencies (#342)
  changes: Correct link to UnboundLocalError fsoids.py fix
  fsrefs: Optimize IO  (take 2) (#340)
  ...

Add support for Python 3.9 and 3.10.

Fix `TypeError: can't concat str to bytes` when running fsoids.py script with Python 3.

Closes #350

fsdump/fsstats improvements

Dieter Maurer notes that loadBefore cannot be deprecated yet because ZEO
essentially depends on the `end_tid` information returned by loadBefore
to update its cache:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-842021970

And to remove this dependency it would require to rework ZODB caching layer:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-845917355

So we cannot deprecate loadBefore until this rework is implemented first.

-> Remove general loadBefore deprecation, and emit loadBefore vs
loadBeforeEx warning only when actually hitting a "deletion" record,
because only that case is known to lead to data corruption.

@d-maurer says (https://github.com/zopefoundation/ZODB/pull/323#discussion_r627929959):

The changes around `loadBeforeEx` have much more impact than the `DemoStorage` fix.

--------

kirr: adjusted the text a bit: "Introduces" -> "Introduce"; add
"interface". @d-maurer, I hope it is ok.

The changelog entry uses loadBeforeEx, but we are likely to change this
name during https://github.com/zopefoundation/ZODB/pull/323 review.

loadBeforeEx uses the same docstring as loadBefore as @d-maurer suggests:

https://github.com/zopefoundation/ZODB/pull/323#discussion_r626255153

@d-maurer suggests to keep loadBefore without deprecation
(https://github.com/zopefoundation/ZODB/pull/323#pullrequestreview-650963363).

-> Don't emit warnings about deprecating loadBefore.

-> Keep the deprecation text in loadBefore interface, since loadBeforeEx
   should practically provide wider functionality without putting
   unnecessary constraint on storage implementations. In other words
   loadBefore deprecation is still there, but less aggressively
   advertised with the idea to make transition for outside-of-ZODB code
   to loadBeforeEx more smooth and with a bit more steps (we might want
   to reinstate the deprecation warnings at a later time).

Suggested by @d-maurer: https://github.com/zopefoundation/ZODB/pull/323#discussion_r625573381

@d-maurer suggests[1]:

    The ZODB logic relating to historical data (including MVCC) was largely
    centered around before. You have changed this to at - requiring wide spread
    modifications. I would much prefer to keep the before centered approach...

    (https://github.com/zopefoundation/ZODB/pull/323#pullrequestreview-650963363)

So let's change "at"-based logic to "before"-based logic and rename the new
method from loadAt to loadBeforeEx.

Define the official expansion of "ZODB"

Switch from Travis CI to GitHub Actions for testing

As suggested by @d-maurer:

https://github.com/zopefoundation/ZODB/pull/345#issuecomment-822188305
https://github.com/zopefoundation/ZEO/issues/168#issuecomment-821829116

/reviewed-on https://github.com/zopefoundation/ZODB/pull/345

For ZEO this data corruption bug was reported at
https://github.com/zopefoundation/ZEO/issues/155 and fixed at
https://github.com/zopefoundation/ZEO/pull/169.

Without that fix the failure shows e.g. as follows when running ZEO test
suite:

    Failure in test check_race_load_vs_external_invalidate (ZEO.tests.testZEO.BlobAdaptedFileStorageTests)
    Traceback (most recent call last):
      File "/usr/lib/python2.7/unittest/case.py", line 329, in run
        testMethod()
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/tests/BasicStorage.py", line 621, in check_race_load_vs_external_invalidate
        self.fail([_ for _ in failure if _])
      File "/usr/lib/python2.7/unittest/case.py", line 410, in fail
        raise self.failureException(msg)
    AssertionError: ['T1: obj1.value (7)  !=  obj2.value (8)']

Even if added test is somewhat similar to
check_race_loadopen_vs_local_invalidate, it is added anew without trying
to unify code. The reason here is that the probability to catch load vs
external invalidation race is significantly reduced when there are only
1 modify and 1 verify workers. The unification with preserving both
tests semantic would make test for "load vs local invalidate" harder to
follow. Sometimes a little copying is better than trying to unify too
much.

For the test to work, test infrastructure is amended with
._new_storage_client() method that complements ._storage attribute:
client-server storages like ZEO, NEO and RelStorage allow several
storage clients to be connected to single storage server. For
client-server storages test subclasses should implement
_new_storage_client to return new storage client that is connected to
the same storage server self._storage is connected to.

For ZEO ._new_storage_client() is added by https://github.com/zopefoundation/ZEO/pull/170

Other client-server storages can follow to implement ._new_storage_client()
and this way automatically activate this "load vs external invalidation"
test when their testsuite is run.

Contrary to test for "load vs local invalidate" N is set to lower value (100),
because with 8 workers the bug is usually reproduced at not-so-high iteration
number (5-10-20).

/cc @d-maurer, @jamadden, @jmuchemb
/reviewed-on https://github.com/zopefoundation/ZODB/pull/345

Add test that exercises open vs invalidation race condition that, if
happen, leads to data corruption. We are seeing such race happening on
storage level in ZEO (https://github.com/zopefoundation/ZEO/issues/166),
and previously we've seen it also to happen on Connection level
(https://github.com/zopefoundation/ZODB/issues/290). By adding this test
to be exercised wrt all storages we make sure that all storages stay
free from this race.

And it payed out. Besides catching original problems from
https://github.com/zopefoundation/ZODB/issues/290 and
https://github.com/zopefoundation/ZEO/issues/166 , this test also
discovered a concurrency bug in MVCCMappingStorage:

    Failure in test check_race_open_vs_invalidate (ZODB.tests.testMVCCMappingStorage.MVCCMappingStorageTests)
    Traceback (most recent call last):
      File "/usr/lib/python2.7/unittest/case.py", line 329, in run
        testMethod()
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/tests/BasicStorage.py", line 492, in check_race_open_vs_invalidate
        self.fail(failure[0])
      File "/usr/lib/python2.7/unittest/case.py", line 410, in fail
        raise self.failureException(msg)
    AssertionError: T1: obj1.value (24)  !=  obj2.value (23)

The problem with MVCCMappingStorage was that instance.poll_invalidations
was correctly taking main_lock with intention to make sure main data is
not mutated during analysis, but instance.tpc_finish and
instance.tpc_abort did _not_ taken main lock, which was leading to
committed data to be propagating into main storage in non-atomic way.

This bug was also observable if both obj1 and obj2 in the added test
were always loaded from the storage (added obj2._p_invalidate after
obj1._p_invalidate).

-> Fix MVCCMappingStorage by correctly locking main MVCCMappingStorage
instance when processing transaction completion.

/cc @d-maurer, @jamadden, @jmuchemb
/reviewed-on https://github.com/zopefoundation/ZODB/pull/345

buildout: Fix CI

/reviewed-on https://github.com/zopefoundation/ZODB/pull/346
/reviewed-by @d-maurer

Commit 75c703fc (Security fix documentation dependencies (#342)) changed
requirements for pygments from

    # pygments 2.6 stops the support for python2
    pygments<2.6

to

    # Silence dependabot claiming a security issue in older versions:
    pygments >= 2.7.4

However this broke CI for python2, as sphinx now fails to install:

    Collecting Sphinx
      Using cached Sphinx-1.8.5-py2.py3-none-any.whl (3.1 MB)
    ERROR: Could not find a version that satisfies the requirement pygments>=2.7.4 (from -r doc/requirements.txt (line 3)) (from versions: 0.5, 0.5.1, 0.6, 0.7, 0.7.1, 0.8, 0.8.1, 0.9, 0.10, 0.11, 0.11.1, 1.0, 1.1, 1.1.1, 1.2, 1.2.1, 1.2.2, 1.3, 1.3.1, 1.4, 1.5, 1.6rc1, 1.6, 2.0rc1, 2.0, 2.0.1, 2.0.2, 2.1, 2.1.1, 2.1.2, 2.1.3, 2.2.0, 2.3.0, 2.3.1, 2.4.0, 2.4.1, 2.4.2, 2.5.1, 2.5.2)
    ERROR: No matching distribution found for pygments>=2.7.4 (from -r doc/requirements.txt (line 3))
    The command "if [[ $TRAVIS_PYTHON_VERSION != pypy3* ]]; then pip install --upgrade --requirement doc/requirements.txt; fi" exited with 1.

(see e.g. https://travis-ci.com/github/zopefoundation/ZODB/jobs/499836548)

Try to fix that by bringing back requirement for `pygments < 2.6` on
python2.

It is not only py2 as build on py3 is also affected, e.g.

https://travis-ci.com/github/zopefoundation/ZODB/jobs/499826281

And even though Sphinx is going to be ready for newer docutils,
sphinx_rtd_theme requires docutils<0.17 for now:

https://github.com/readthedocs/sphinx_rtd_theme/blob/2026bb61ddf85a672141127b162d6f9181603725/setup.py#L119-L122

Else it is failing as:

    Got j1m.sphinxautozconfig 0.1.0.
    Getting distribution for 'sphinx_rtd_theme'.
    Got sphinx-rtd-theme 0.5.2.
    Version and requirements information containing docutils:
      Base installation request: 'Sphinx', 'docutils', 'ZODB', 'sphinxcontrib_zopeext', 'j1m.sphinxautozconfig', 'sphinx_rtd_theme'
      Requirement of sphinx_rtd_theme: docutils<0.17
      Requirement of j1m.sphinxautozconfig: docutils
      Requirement of Sphinx: docutils<0.17,>=0.12
    While:
      Installing sphinx.
    Error: There is a version conflict.
    We already have: docutils 0.17.1
    but sphinx-rtd-theme 0.5.2 requires 'docutils<0.17'.

(see e.g. https://travis-ci.com/github/zopefoundation/ZODB/jobs/499760337)

Suggested by @d-maurer.

Fix tiny issue with a remaining colon that was probably not deleted during updates of the README.