Treat `PY3` just as `six.PY3`

[pquentin]

@hugovk submitted a pull request to urllib3 based on this plugin: urllib3/urllib3#1669. Thanks! But it failed to catch all issues, as PY3 was sometimes imported directly. See urllib3/urllib3#2004 for examples.

This may cause false positives, though.

[asottile]

PY3 could be defined correctly as sys.version_info >= (3,) so this isn't really valid -- it does correctly identify six.PY3 and from six import PY3 already

[pquentin]

I see, that makes sense!

For what it's worth, the from six import PY3 detection does not work in the case of urllib3 since we vendor six, but there's not much you can do about it.

[asottile]

correct, we'd need to do dynamic analysis which at best would be slow and at worst would be solving the halting problem