Fix Python 3.5 MacOS wheel upload

Python 3.5 on Mac OS doesn't support TLS 1.2, which makes twine upload
fail, but we can use system Python 2.7 for twine upload, which works.

Also brings the rest of the Travis CI configuration closer in line to
the rest of the packages updated for
https://github.com/zopefoundation/meta/issues/11

Build using newer manylinux image.

And update macOS python to 2.7.17 to try to fix uploads.

Restructure Travis to always build manylinux wheels

This PR makes it so we _always_ build manylinux wheels, but only upload
them for git tags.

The diff is messy, for which I apologize.  I had to reformat the YAML to
understand it.  Highlights:

- Moved regular Linux Python builds outside build matrix, for
  conciseness (unfortunately this means 3.6 builds are now done twice:
  once without WITH_COVERAGE and once with)

- Move twine upload from .manylinux.sh into .travis.yml's after_success,
  next to the twine upload used for Mac OS builds

- This means Travis can no longer do an 'exit 0' in before_install, so I
  had to override the install and script sections too

- Move 'docker pull' into the install section so it's not so very empty

- Unify the two calls to twine upload to provide the twine password via
  environment variables instead of command-line or configuration files

- Put related commands inside a single multiline if statement instead of
  repeating the conditions

Add support for Python 3.8

I'm skipping the MacOS build on Travis because 3.8 is not available yet
from https://github.com/MacPython/terryfy

Some documentation love

Fix segfault on repr(OOBucket([... non-Latin-1 thingies ...]))

Fixes #106.

The bug was in not checking the return value of
PyUnicode_AsLatin1String() and passing a NULL to PyOS_snprintf().

The fix is to use PyUnicode_FromFormat() and avoid all the hassle of
manual encoding/sprintf/decoding and even calling repr().

The test fails on Python 3 only, and on non-pure builds only.  I added
PYTHONFAULTHANDLER=1 to the environment to make the failure less
mysteriously silent.

I've also updated tox.ini to use extras=foo instead of deps=.[foo] and
to use factors to set PURE_PYTHON=1 instead of defining an entire
environment.  This now lets you run arbitrary pure Python builds like
tox -e py37-pure.

Drop Python 3.4 support

Stop installing terryfy.

Update tests for persistent 4.4.3

Fix tests against persistent 4.4.

Stop being quite so strict to account for the differences between the
C and Python implementations. But do test that we don't break the new
features when we didn't already have a better form.

Fixes #94

Refs https://github.com/zopefoundation/persistent/issues/92

Avoid spurious "failures" on Appveyor during releases

This way if the sdist hasn't been uploaded yet for some reason, Appveyor
will take care of it.

PowerShell interprets any stderr output from a child process as an
error.  This means warnings from pip about using
not-the-very-latest-version-how-dare-you and warnings from twine about
uploading the *.tar.gz which was probably already uploaded already and
might not be a bitwise identical copy because what even is
reproducibility cause the Appveyor builds to fail, even if the actual
binary wheels we want have been built and uploaded successfully.

*pauses to breathe*

[skip ci]

[skip ci]