python: implement reverse lookup function

The implementation follows for most part the PHP code but introduces an
additional layer parameter with which the kind of places to be returned
can be restricted. This replaces the hard-coded exclusion lists.

nominatim/api/lookup.py

@@ -172,6 +172,7 @@ async def get_place_by_id(conn: SearchConnection, place: ntyp.PlaceRef,
     assert result is not None
     result.parent_place_id = row.parent_place_id
     result.linked_place_id = getattr(row, 'linked_place_id', None)
+    result.admin_level = getattr(row, 'admin_level', 15)
     indexed_date = getattr(row, 'indexed_date', None)
     if indexed_date is not None:
         result.indexed_date = indexed_date.replace(tzinfo=dt.timezone.utc)