py3: add warnings in check-code related to py3
We have our own bytes versions of things like, getopt.getopt, os.sep, os.name,
sys.executable, os.environ and few more for python 3 portability. Its better
to come up with warnings if someone breaks the things which we have fixed.
After this patch, check-code will warn us to use our bytes version.
These checks run on mercurial/ and hgext/ and pycompat.py is excluded.
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).