docs: def _watch_file(self, filepath, trigger_event=True):
          """Adds the file's modified time into its internal watchlist."""
          is_new = filepath not in self._watched_files
          if trigger_event:
              if is_new:
                  self.trigger_created(filepath)
              else:
                  self.trigger_modified(filepath)
          try:
              self._watched_files[filepath] = self._get_modified_time(filepath)
          except OSError:
              return