Fix for Python 4

[hugovk]

Summary of changes

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but it will probably happen in 2020 (when Python 3.9 reaches beta).

six.py defines these:

PY2 = sys.version_info[0] == 2
PY3 = sys.version_info[0] == 3

There's several places in the codebase like:

if six.PY3:
    pass  # Python 3+ stuff

if not six.PY3:
    pass  # Python 2 stuff

But when run on Python 4, it will skip the Python 3+ stuff and run the Python 2 stuff!

This PR flips it around:

if not six.PY2:
    pass  # Python 3+ stuff

if six.PY2:
    pass  # Python 2 stuff

Found using:

$ pip install -U flake8-2020
...
$ flake8 --select YTT
./dateutil/test/test_parser.py:465:16: YTT202: `six.PY3` referenced (python4), use `not six.PY2`
./dateutil/test/test_rrule.py:2287:16: YTT202: `six.PY3` referenced (python4), use `not six.PY2`
./dateutil/test/test_rrule.py:4582:16: YTT202: `six.PY3` referenced (python4), use `not six.PY2`
./dateutil/test/test_isoparser.py:309:20: YTT202: `six.PY3` referenced (python4), use `not six.PY2`
./dateutil/test/test_isoparser.py:359:8: YTT202: `six.PY3` referenced (python4), use `not six.PY2`

Pull Request Checklist

• Changes have tests
• Authors have been added to AUTHORS.md
• News fragment added in changelog.d. See CONTRIBUTING.md for details

[pganssle]

Can you add a news fragment in changelog.d?

It should be named 950.bugfix.rst. The text can be something like:

Changed compatibility logic to support a potential Python 4.0 release. Patch by Hugo van Kemenade (gh pr #950)

[hugovk]

Added!