formatting: using black to check for formatting

formatting: run black version 19.10b0 on the codebase The latest version of black is out and contains the change we needed. So we can start using it now. note: `test-check-format.t` will complains about this changes because it still use `grey` and need to be migrated to `black`. See next changesets for this.

packaging: fix buildrpm whitespace

packaging: drop outdated buildrpm "tested on" comment Packaging usually works on other versions too, and it is not efficient to maintain the list in repo. It is already out of sync with the Makefile targets.

packaging: also include hgweb.wsgi in rpms

packaging: introduce Python3 support as buildrpm --python3 Just overrule the HGPYTHON3 warning.

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.