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.