setup.cfg: Support flake8-future-import 0.4
setup.cfg: Support flake8-future-import 0.4

flake8-future-import added some older __future__ methods
which are before python 2.6. So, we need to specify which python
version we plan to support so that those are ignored by default.

AbdealiJK/file-metadata,AbdealiJK/file-metadata,pywikibot-catfiles/file-metadata,pywikibot-catfiles/file-metadata