Core: fix some memory leak sources without removing caching (#2400)

* Core: fix some memory leak sources

* Core: run gc before detecting memory leaks

* Core: restore caching in BaseClasses.MultiWorld

* SM: move spheres cache to MultiWorld._sm_spheres to avoid memory leak

* Test: add tests for world memory leaks

* Test: limit WorldTestBase leak-check to py>=3.11

---------

Co-authored-by: Fabian Dill <fabian.dill@web.de>

test/general/test_memory.py

@@ -0,0 +1,16 @@
+import unittest
+
+from worlds.AutoWorld import AutoWorldRegister
+from . import setup_solo_multiworld
+
+
+class TestWorldMemory(unittest.TestCase):
+    def test_leak(self):
+        """Tests that worlds don't leak references to MultiWorld or themselves with default options."""
+        import gc
+        import weakref
+        for game_name, world_type in AutoWorldRegister.world_types.items():
+            with self.subTest("Game", game_name=game_name):
+                weak = weakref.ref(setup_solo_multiworld(world_type))
+                gc.collect()
+                self.assertFalse(weak(), "World leaked a reference")