Encode datetime.datetime.

def _encode_datetime(name, value, dummy0, dummy1):
    """Encode datetime.datetime."""
    if value.utcoffset() is not None:
        value = value - value.utcoffset()
    millis = int(calendar.timegm(value.timetuple()) * 1000 +
                 value.microsecond / 1000)
    return b"\x09" + name + _PACK_LONG(millis)