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.
This was previously D9833, but was rolled back due to test
breakage. nbjoerg thinks that breakage is now resolved, so we're
trying again.
Differential Revision: https://phab.mercurial-scm.org/D10184
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).