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
13 jobs for topic/default/netio-tests in 26 minutes and 11 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Tests
passed #170665
checks-py2

00:03:38

passed #170666
checks-py3

00:03:16

passed #170667
rust-cargo-test-py2

00:01:45

passed #170668
rust-cargo-test-py3

00:02:17

passed #170669
test-py2

00:20:53

passed #170675
test-py2-chg

00:20:56

passed #170671
test-py2-pure

00:19:49

passed #170673
test-py2-rust

00:25:25

passed #170670
test-py3

00:15:29

passed #170676
test-py3-chg

00:13:31

passed #170672
test-py3-pure

00:18:25

passed #170674
test-py3-rust

00:14:16

 
  Phabricator
passed #170677
phabricator-refresh

00:00:44