setup: allow to run setup.py with python 3 without a mercurial checkout Some people may want to test mercurial in a python 3 environment through e.g. pip, in which case setup.py doesn't run in a mercurial checkout, so the hack in setup.py to allow python 3 cannot be overcome. This change allows a manual override with the HGPYTHON3 environment variable. Additionally, when for some reason the version is unknown (for crazy people like me, who have a git checkout of the mercurial repo), the version variable ends up being an unicode string, which fails the `isinstance(version, bytes)` assertion. So fix that at the same time. Differential Revision: https://phab.mercurial-scm.org/D3958