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

setup.py

@@ -74,7 +74,7 b' if sys.version_info[0] != 2:'
     badpython = True
 
     # Allow Python 3 from source checkouts.
-    if os.path.isdir('.hg'):
+    if os.path.isdir('.hg') or 'HGPYTHON3' in os.environ:
         badpython = False
 
     if badpython:
@@ -369,7 +369,7 b' try:'
     from mercurial import __version__
     version = __version__.version
 except ImportError:
-    version = 'unknown'
+    version = b'unknown'
 finally:
     if oldpolicy is None:
         del os.environ['HGMODULEPOLICY']