Replace some isinstance calls with TypeGuard.

These aren't as ergonomic as I'd hoped because of
https://www.python.org/dev/peps/pep-0647/#narrowing-of-implicit-self-and-cls-parameters.

I added a decorator `@self_type_guard` so we can avoid needing to import
the descendent types in the typeguard implementation (which wouldn't fix
any circular imports, just move them).

game/typeguard.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from typing import Callable, TypeGuard, TypeVar
+
+SelfT = TypeVar("SelfT")
+BaseT = TypeVar("BaseT")
+GuardT = TypeVar("GuardT")
+
+
+def self_type_guard(
+    f: Callable[[SelfT, BaseT], TypeGuard[GuardT]]
+) -> Callable[[SelfT, BaseT], TypeGuard[GuardT]]:
+    def decorator(s: SelfT, arg: BaseT) -> TypeGuard[GuardT]:
+        if id(s) != id(arg):
+            raise ValueError(
+                "self type guards must be called with self as the argument"
+            )
+        return f(s, arg)
+
+    return decorator