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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true