Core: fix settings API for removal of Python 3.8, 3.9 (#4280)

* Core: fix settings API for removal of Python 3.8, 3.9

This is fixing 2 problems:
- The `World` class has the annotation:
  `settings: ClassVar[Optional["Group"]]`
  so `MyWorld.settings` should not raise an exception like it does for some worlds.
  With the `Optional` there, it looks like it should return `None` for the worlds that don't use it. So that's what I changed it to.

- `Group.update` had some code that required `typing.Union` instead of the Python 3.10 `|` for unions.

added unit test for this fix
added change in Zillion that I used to discover this problem and used it to test the test

* fix copy-pasted stuff

* tuple instead of set

Co-authored-by: black-sliver <59490463+black-sliver@users.noreply.github.com>

---------

Co-authored-by: black-sliver <59490463+black-sliver@users.noreply.github.com>

test/general/test_settings.py

@@ -0,0 +1,16 @@
+from unittest import TestCase
+
+from settings import Group
+from worlds.AutoWorld import AutoWorldRegister
+
+
+class TestSettings(TestCase):
+    def test_settings_can_update(self) -> None:
+        """
+        Test that world settings can update.
+        """
+        for game_name, world_type in AutoWorldRegister.world_types.items():
+            with self.subTest(game=game_name):
+                if world_type.settings is not None:
+                    assert isinstance(world_type.settings, Group)
+                    world_type.settings.update({})  # a previous bug had a crash in this call to update