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/**"'`