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 shot. Differential Revision: https://phab.mercurial-scm.org/D10185