**Summary** Implement docstring formatting
**Test Plan** Matches black's `docstring.py` fixture exactly, added some
new cases for what is hard to debug with black and with what black
doesn't cover.
similarity index:
main:
zulip: 0.99702
django: 0.99784
warehouse: 0.99585
build: 0.75623
transformers: 0.99469
cpython: 0.75989
typeshed: 0.74853
this branch:
zulip: 0.99702
django: 0.99784
warehouse: 0.99585
build: 0.75623
transformers: 0.99464
cpython: 0.75517
typeshed: 0.74853
The regression in transformers is actually an improvement in a file they
don't format with black (they run `black examples tests src utils
setup.py conftest.py`, the difference is in hubconf.py). cpython doesn't
use black.
Closes#6196
