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>

settings.py

@@ -7,6 +7,7 @@ import os
 import os.path
 import shutil
 import sys
+import types
 import typing
 import warnings
 from enum import IntEnum
@@ -162,8 +163,13 @@ class Group:
             else:
                 # assign value, try to upcast to type hint
                 annotation = self.get_type_hints().get(k, None)
-                candidates = [] if annotation is None else \
-                    typing.get_args(annotation) if typing.get_origin(annotation) is Union else [annotation]
+                candidates = (
+                    [] if annotation is None else (
+                        typing.get_args(annotation)
+                        if typing.get_origin(annotation) in (Union, types.UnionType)
+                        else [annotation]
+                    )
+                )
                 none_type = type(None)
                 for cls in candidates:
                     assert isinstance(cls, type), f"{self.__class__.__name__}.{k}: type {cls} not supported in settings"