Remove circular reference from OverloadedDict

Now KeyOverloads only have a proxy reference to the parent object. This prevents these objects being managed by the generational garbage collector. That was bad since it could keep large AnnData objects in memory. See scverse#360 for more info.

anndata/compat/_overloaded_dict.py

@@ -2,6 +2,7 @@
 from functools import partial
 from typing import Any, Callable, Mapping, Optional
 from warnings import warn
+from weakref import proxy
 
 
 class KeyOverload:
@@ -90,7 +91,7 @@ def __init__(self, data: Mapping, *, overloaded: Mapping[Any, KeyOverload]):
         self.data = data
         self.overloaded = overloaded
         for v in overloaded.values():
-            v.parent = self
+            v.parent = proxy(self)
 
     def __getitem__(self, key):
         if key in self.overloaded: