flake8 ignores @python2 dir

[Akuli]

We should probably remove it after this PR, since I expect many new errors.

Originally posted by @srittau in #5451 (comment)

[JelleZijlstra]

Are there more errors after your round of unused import fixes?

[Akuli]

Besides unused imports fixed in #5453 , there were no errors. I think this is because most Python 2 stubs are copies of flake8 checked Python 3 stubs.