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.