Pre-emptive Python 3.10 compat.

Iterator is no longer exposed directly in collections, but is in
collections.abc.

game/sim/aircraftsimulation.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import itertools
-from collections import Iterator
+from collections.abc import Iterator
 from datetime import datetime, timedelta
 
 from typing_extensions import TYPE_CHECKING