tests: verify that Python scripts are blackened
Scripts are Python too. They should be blackened.
(As suggested by Pierre-Yves in D7420.)
Differential Revision: https://phab.mercurial-scm.org/D7447
#require black
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`