Fix for Python 4: replace unsafe six.PY3 with PY2

[hugovk]

Summary of changes

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but whichever it is, it will probably happen in 2020 when Python 3.9 reaches beta and work begins on Python 3.9+1.

There's some code which uses six.PY3:

if six.PY3:
    print("Python 3+ code")
else:
    print "Python 2 code"

Where:

PY3 = sys.version_info[0] == 3

When run on Python 4, this will run the Python 2 code! Instead, use six.PY2.

Found using https://github.com/asottile/flake8-2020

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[jaraco]

Perhaps this issue should be fixed in six rather than in every package on PyPI. I'm pretty sure PY3 was never meant to mean PY<4.

[hugovk]

It's a WONTFIX in six.

benjaminp/six#297

[Harmon758]

Note, it's almost certainly going to be 3.10. That's the consensus from the Python-Dev mailing list and that's what's being used to reference the future release in PEPs and CPython. There's even references to 3.11 and 3.12 already.

[hugovk]

Thanks for the link!

[webknjaz]

Found using asottile/flake8-2020

Maybe this should also be added to tests/requirements.txt

[hugovk]

Good idea, please see PR #1968.