setup: explicitly declare supported Python versions I think we should probably backport this to 4.2 as well, and do one more release there that explicitly declares 2.6 support. That way anyone stuck on Python 2.6 will end up getting the right hg if they use a modern pip to install. Users can still use `python setup.py` incantations to attempt installing Mercurial on unsupported Pythons, including 3.5 and 3.6. A followup change will switch to only doing our own Python-version-check logic if we're not being installed by a reasonable pip.

contrib: remove one test from the python 3 whitelist 439b4d005b4a introduced a use of 'hg revert --no-backup' which is currently broken. Rather than try and do a fix on the (complicated) revert code during the freeze, let's just give up on this test under Python 3 until later. Once we do fix things, the test ratchet script will re-enable it.

ui: fix configbytes isinstance check to look for bytes and not str Fixes configbytes on Python 3.