black: merge config into main pyproject.toml now that we have it
This means that naive contributors who just run `black` on a source file
will get reasonable behavior as long as they have a recent black. Yay!
Differential Revision: https://phab.mercurial-scm.org/D9834
pyproject: add config file
This will tell pip et al to call our setup.py for the majority of
packaging concerns, but also gives us a place to put standard config
stuff like black.
Differential Revision: https://phab.mercurial-scm.org/D9833