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!
This was previously D9834 but was rolled back due to test
failures. nbjoerg thinks it's time to try again, so let's give it a
Differential Revision: https://phab.mercurial-scm.org/D10185