check-py3-compat: drop support for Python 2
We no longer support Python 2 so we can drop support for linting code
for Python 2 support.
This gets rid of the check for `from __future__`, enabling us to delete
those imports.
Differential Revision: https://phab.mercurial-scm.org/D12251
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).