Configuring for pure-python

[mgedmin]

(lol I've no idea what I'm doing)

TODO:

• [x] fix lint errors
• [x] specify a minimum coverage number
• [x] mention Python 3.9 support in the changelog
• [x] fix concurrency bug in test

[mgedmin]

BTW I once saw a nondeterministic failure while running tox -p auto:

✔ OK py36 in 2.412 seconds
ERROR: invocation failed (exit code 1), logfile: /home/mg/src/zopefoundation/zope.testing/.tox/py35/log/py35-7.log
==================================================================================================== log start ====================================================================================================
py35 develop-inst-noop: /home/mg/src/zopefoundation/zope.testing
py35 installed: six==1.15.0,zope.exceptions==4.4,zope.interface==5.2.0,-e git+git@github.com:zopefoundation/zope.testing.git@26ee1de88cfee213dad2d36bd723d50ef963a499#egg=zope.testing,zope.testrunner==5.2
py35 run-test-pre: PYTHONHASHSEED='1124212290'
py35 run-test: commands[0] | zope-testrunner --test-path=src
Running zope.testrunner.layer.UnitTests tests:
  Set up zope.testrunner.layer.UnitTests in 0.000 seconds.

Failure in test /home/mg/src/zopefoundation/zope.testing/src/zope/testing/setupstack.txt
Failed doctest test for setupstack.txt
  File "/home/mg/src/zopefoundation/zope.testing/src/zope/testing/setupstack.txt", line 0

----------------------------------------------------------------------
File "/home/mg/src/zopefoundation/zope.testing/src/zope/testing/setupstack.txt", line 262, in setupstack.txt
Failed example:
    os.remove('t')
Exception raised:
    Traceback (most recent call last):
      File "/home/mg/opt/python35/lib/python3.5/doctest.py", line 1321, in __run
        compileflags, 1), test.globs)
      File "<doctest setupstack.txt[46]>", line 1, in <module>
        os.remove('t')
    FileNotFoundError: [Errno 2] No such file or directory: 't'

  Ran 21 tests with 1 failures, 0 errors and 0 skipped in 0.160 seconds.
Tearing down left over layers:
  Tear down zope.testrunner.layer.UnitTests in 0.000 seconds.
ERROR: InvocationError for command /home/mg/src/zopefoundation/zope.testing/.tox/py35/bin/zope-testrunner --test-path=src (exited with code 1)

===================================================================================================== log end =====================================================================================================
✖ FAIL py35 in 2.437 seconds

I think the test suite is mildly buggy. I'll try to fix it.

[mgedmin]

Looked at the diff, saw an exclude appveyor.yml in the old MANIFEST.in, updated ,meta.toml to do the same.

[mgedmin]

Reconfigured --with-appveyor, let's see what happens now.