black: make codebase compatible with black v21.4b2 and v20.8b1
I don't know what exact version of black made it care about these whitespace
differences, but this is the version I got when I just installed it with
pip3 install black.
I'm intentionally not increasing the version of black required, as I don't want
to force everyone to upgrade their version of black, and these fixes are
backwards compatible with black v20.8b1. If there are more issues in the future
and this becomes a maintenance burden I may do so in a future change.
Tested with both versions of black (I got the older version via
pip3 install black==20.8b1)
Differential Revision: https://phab.mercurial-scm.org/D10539