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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def last_modified(self) -> Optional[datetime.datetime]: """The value of Last-Modified HTTP header, or None. This header is represented as a `datetime` object. """

        httpdate = self._headers.get(hdrs.LAST_MODIFIED)
        if httpdate is not None:
            timetuple = parsedate(httpdate)
            if timetuple is not None:
                return datetime.datetime(*timetuple[:6],
                                         tzinfo=datetime.timezone.utc)
        return None