fsmonitor: remove pywatchman from exclusion rule The recently vendored pywatchman code base is now formatted with black. We can now remove pywatchman from our black exclusion rule. Differential Revision: https://phab.mercurial-scm.org/D7202

#require black

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