docs: def _sorted_keys(self):
          """
          Return list of keys sorted by version
  
          Sorting is done based on :py:func:`pkg_resources.parse_version`
          """
  
          try:
              keys = self._cache['sorted_keys']
          except KeyError:
              keys = self._cache['sorted_keys'] = sorted(self.keys(), key=parse_version)
  
          return keys