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

def request_timestamp(self, lock):
        '''Return the timestamp of our outstanding request for lock, or None.

        Returns a datetime.datetime() UTC timestamp, with no tzinfo attribute.
        '''
        ts = self.requests[hookenv.local_unit()].get(lock, None)
        if ts is not None:
            return datetime.strptime(ts, _timestamp_format)