tests: use TypeAlias for Fixtures instead of subclass

I don't remember why I did it that way in the first place.

tests/fixture.py

@@ -38,10 +38,7 @@
 now = partial(dt.datetime.now, tz=dt.UTC)
 
 
-class Fixtures(SimpleNamespace):
-    pass
-
-
+Fixtures: TypeAlias = SimpleNamespace
 FixtureOptions: TypeAlias = dict[str, Any]
 FixtureContext: TypeAlias = Iterator
 FixtureFunction: TypeAlias = Callable[[FixtureOptions, Fixtures], Any]