packaging: be explicit about Python version in rpm spec
Fedora 31 has Python3 at /usr/bin/python ... but expect everybody to not just
find python in $PATH but be explicit about whether they want python2 or
python3. mercurial.spec just used 'python' and would fail when it unknowingly
used Python 3 and ended up with Mercurial setup.py reporting "Python 3.7
detected." and talking about the HGPYTHON3 environment variable.
For now, just be explicit about using system python2 as python executable when
building rpms.
packaging: make python snippets in rpm building python3 compatible
Fedora 31 has Python3 at /usr/bin/python, and buildrpm would fail on snippets
that use python2 syntax. Instead of forcing python2, just accept for the future
while staying backwards compatible.