Fixes #10158.

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  (edit by @plamut: opened the issue for better traceability)
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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.