mypy: Turn on return value checks

Its a shame GvR is married to "return None"
https://github.com/python/mypy/issues/7511

kitty/layout/grid.py

@@ -81,6 +81,7 @@ class Grid(Layout):
                 if idx == window_idx:
                     return row_num, col_num
                 row_num += 1
+            return 0, 0
 
         row_num, col_num = position_for_window_idx(idx)