Use directly check() api from pydocstyle.

def run(self):
    """Use directly check() api from pydocstyle."""
    if self.exclude_from_doctest:
      for pattern in self.exclude_from_doctest:
        if fnmatch.fnmatch(self.filename, pattern):
          return

    checked_codes = pep257.conventions.pep257 | {'D998', 'D999'}
    for error in self._check_source():
      if isinstance(error, pep257.Error) and error.code in checked_codes:
        # NOTE(sigmavirus24): Fixes GitLab#3
        message = '%s %s' % (error.code, error.short_desc)
        yield (error.line, 0, message, type(self))