automation: support Python 3.10 on Windows
Python 3.10 is out and we should support it. This commit teaches the
automation code to install and support building for Python 3.10.
Differential Revision: https://phab.mercurial-scm.org/D11776
rhg: implement the debugignorerhg subcommand
This can be used to inspect the generated pattern, but also to benchmark
the time it takes to parse hgignore.
Differential Revision: https://phab.mercurial-scm.org/D11722