tests/test-check-format.t

contrib: require Python 3.6 for byteify-strings.py

This script makes use of `token.COMMENT`, which apparently
isn't present until Python 3.6. So make the script and its
test conditional on Python 3.6.

#require black

  $ cd $RUNTESTDIR/..
  $ black --config=black.toml --check --diff `hg files 'set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`