<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _parse_expiry(response_data):
 """Parses the expiry field from a response into a datetime. Args: response_data (Mapping):
 The JSON-parsed response data. Returns: Optional[datetime]: The expiration or ``None`` if no expiration was specified. """

    expires_in = response_data.get('expires_in', None)

    if expires_in is not None:
        return _helpers.utcnow() + datetime.timedelta(
            seconds=expires_in)
    else:
        return None