fix: replace unsafe six.PY3 with PY2 for better future compatibility with Python 4

[hugovk]

Fixes #10158.

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---

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.

[plamut]

Testing for "is (not) PY2" instead of "is (not) PY3" is indeed more robust, thanks for catching that!

The change is done to an auto-generated file

[plamut]

(speaking for the PubSub library)

The change is good, but needs to be made in the synth file that is used to generate the file changed here.

The changes to the files in other libraries are fine, as those files are not auto-generated (AFAIK).

[plamut]

LGTM with one final remark.

[plamut]

LGTM. Thanks @hugovk!

Will delay merging for a day or so if anybody else wants to review it.

[plamut]

The test failures are not related, also happen on master. Investigating.

Update: At least for logging, the test failure is caused by an out-of-date assertion - an optional timeout was added to core recently. Will be fixed in #10087.

Update 2: The Storage failures also seem to be a regression, will submit a fix for that, too (edit: submitted).