hgview #75296 remove mercurial as a setup.py-time dependency [validation pending]

I don't see any reason why a package build system should have mercurial installed just to package this package.

Mercurial is apparently only needed because setup.py uses hgviewlib/__pkginfo__.py and hgviewlib/__init__.py monkey patches Mercurial. It must be possible to do that elsewhere or differently.

done in1.5.0
load left0.000
closed by#80a177693edc [hg patches] Dedicate a module to patch Mercurial (closes #75296)