formating: upgrade to black 20.8b1
This required a couple of small tweaks to un-confuse black, but now it
works. Big formatting changes come from:
* Dramatically improved collection-splitting logic upstream
* Black having a strong (correct IMO) opinion that """ is better than '''
Differential Revision: https://phab.mercurial-scm.org/D9430
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support* (glob)
Ran 1 tests, 1 results as expected* (glob)