Make sure that loadBefore actually uses ._conflict_cache data

TemporaryStorage uses

    ._index   {} oid -> serial
    ._opickle {} oid -> data

as data store for load. However for loadBefore

    ._conflict_cache  {} (oid,serial) -> (data, time)

is reused as data store.

That would be ok if that place would be treated as data store, but given
its primary purpose - as its name suggests - was originally to be a
cache to resolve conflicts, it is "logical" that entries in this cache
are garbage-collected when entry age becomes > gc threshold.

Only now there is a problem: if an object is committed once, and time
passes, corresponding entry in ._conflict_cache will be removed. And this
would manifest itself as

    - load(oid)  -> gives latest data for the object (obtained via ._index and ._opickle)

but

    - loadBefore(oid, @head)  -> gives POSKeyError

-> Fix it by always preserving latest object revision in
._conflict_cache from being removed on GC.

The fix is important for systems that use ZODB5, or ZODB4-wc2[1] because
there ZODB.Connection switched from primarily using load to exclusively
using loadBefore.

/cc @icemac @mauritsvanrees @mgedmin @d-maurer @dwt @hannosch

Fixes: https://github.com/zopefoundation/tempstorage/issues/8
[1] nexedi/ZODB@8e7eab33

Fix CI

- Fix tests for ZODB5.6
- Switch from Travis to GitHub actions

See individual commits for details.

/reviewed-by @dataflake 
/reviewed-on https://github.com/zopefoundation/tempstorage/pull/17

- Travis stopped providing unlimited free minutes to free software projects,
- Most zopefoundation/* projects already switch to GitHub actions for that reason,
- Despite tempstorage being dormant for a long time with no push/PR
  activity, Travis did not run any tests at all for https://github.com/zopefoundation/tempstorage/pull/16

By definition IStorage level methods need to be passed in storage-level
IStorageTransactionMetaData, not regular transaction.Transaction
objects:

https://github.com/zopefoundation/ZODB/blob/5.6.0-35-g1fb097b41/src/ZODB/interfaces.py#L827-L830

Passing in there regular transaction.Transaction used to work
historically, but after https://github.com/zopefoundation/ZODB/commit/2f8cc67a
it started to fail as:

    Error in test test_conflict_cache_clears_over_time (tempstorage.tests.testTemporaryStorage.TemporaryStorageTests)
    Traceback (most recent call last):
      File "/usr/lib/python2.7/unittest/case.py", line 329, in run
        testMethod()
      File "/home/kirr/src/wendelin/z/tempstorage/src/tempstorage/tests/testTemporaryStorage.py", line 195, in test_conflict_cache_clears_over_time
        self._dostore(storage, oid1, data=MinPO(5))
      File "/home/kirr/src/wendelin/z/tempstorage/src/tempstorage/tests/testTemporaryStorage.py", line 132, in _dostore
        storage.tpc_begin(t)
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/BaseStorage.py", line 193, in tpc_begin
        ext = transaction.extension_bytes
    AttributeError: 'Transaction' object has no attribute 'extension_bytes'

-> Fix it by using storage-level transactions in tests.

As requested in https://github.com/zopefoundation/meta/issues/116

* Document deprecation of this package.
* Document suggested alternatives.
* Warn at runtime about the deprecation status of the package so developers using it can get a hint even when they don't read the readme of this package.

Document deprecation of this package.

Apply suggestion
Co-Authored-By: Michael Howitz <mh@gocept.com>

* fix RuntimeError in py3
* move test requirements from tox.ini to setup.py

[skip ci]

modified:   .travis.yml
modified:   CHANGES.rst
modified:   setup.py
modified:   tox.ini

modified:   .travis.yml
modified:   CHANGES.rst
modified:   setup.py
modified:   tox.ini

cf https://docs.travis-ci.com/user/languages/python/#pypy-support

This closes #9.

modified:   .travis.yml
modified:   CHANGES.rst

... by installing an updated setuptools version.

modified:   .travis.yml

This prevents a KeyError: '\x00\x00\x00\x00\x00\x00\x00\x00'

Removed doubt-inducing version pins