Small optimisation of YTT202

[hugovk]

With this code:

import six
if not six.PY3:
    pass

The plugin correctly gives:

$ flake8 --select YTT 1.py
1.py:2:8: YTT202: `six.PY3` referenced (python4), use `not six.PY2`

Perhaps it could detect the not before six.PY3, and suggest a clean six.PY2 instead?

For example:

$ flake8 --select YTT 1.py
1.py:2:X: YTT20X: `not six.PY3` referenced (python4), use `six.PY2`

That is:

# Bad
if not six.PY3:

# Better (sort of)
if not not six.PY2:

# Best
if six.PY2:

[asottile]

yeah it would be better, but it'd be very hard to implement -- and since this isn't a code formatter I'm inclined to let the human do the right thing 🤷‍♂