tests/test-check-format.t

win32mbcs: fix a `str` type conditional for py3

Differential Revision: https://phab.mercurial-scm.org/D7535

#require black

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