Test on Python 3.9-dev

[hugovk]

Python 3.9 is currently in alpha with a beta due in a couple of weeks (2020-05-18).

• https://www.python.org/dev/peps/pep-0596/#schedule

This PR tests 3.9-dev on Travis CI, currently Python 3.9.0a6+.

However, it's failing, so as a pre-release I put it in the allow_failures so it won't turn the whole CI red.

The error is:

======================================================================
ERROR: tests.test_black (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: tests.test_black
Traceback (most recent call last):
  File "/opt/python/3.9-dev/lib/python3.9/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/opt/python/3.9-dev/lib/python3.9/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/home/travis/build/hugovk/black/tests/test_black.py", line 20, in <module>
    import black
  File "/home/travis/build/hugovk/black/black.py", line 140, in <module>
    TMatchResult = TResult[Index]
  File "/opt/python/3.9-dev/lib/python3.9/typing.py", line 244, in inner
    return func(*args, **kwds)
  File "/opt/python/3.9-dev/lib/python3.9/typing.py", line 704, in __getitem__
    arg = arg[subargs]
  File "/opt/python/3.9-dev/lib/python3.9/typing.py", line 244, in inner
    return func(*args, **kwds)
  File "/opt/python/3.9-dev/lib/python3.9/typing.py", line 695, in __getitem__
    _check_generic(self, params)
  File "/opt/python/3.9-dev/lib/python3.9/typing.py", line 194, in _check_generic
    raise TypeError(f"{cls} is not a generic class")
TypeError: black.Err[black.CannotTransform] is not a generic class

https://travis-ci.org/github/hugovk/black/jobs/684724917

[JelleZijlstra]

Thanks! The error looks like a CPython bug; I'll try to distill it into a bug report unless somebody else gets to it.

[hugovk]

Oh, good to find it :)

@brettcannon will be happy, I was following his suggestion of testing dev versions!

https://snarky.ca/how-to-use-your-project-travis-to-help-test-python-itself/

[JelleZijlstra]

Posted in https://bugs.python.org/issue40397

[cooperlees]

I might put a PR up to see if anyone else agrees.

[cooperlees]

I'm more of a fan of nightly for failure allowed testing:

    - python: nightly
  allow_failures:
    - python: nightly

Then we get signal on the net versions fixes sooner :)

[hugovk]

Is it so that nightly and 3.9-dev are the same thing at the moment? And at some point in the future, nightly will be 3.10?

Sounds like a good idea to add it.

I'd suggest also keeping 3.9-dev as a gating check which can fail the whole build, ie. not in allowed_failures, to make sure new PRs pass on 3.9. (But only when the CPython bug is fixed and released.)