<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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