gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible (#136893)

An interesting hack, but more localized in scope than #135230.

This may be a breaking change if people intentionally keep the original class around
when using `@dataclass(slots=True)`, and then use `__dict__` or `__weakref__` on the
original class.

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>

Lib/dataclasses.py

@@ -1338,6 +1338,13 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
                 or _update_func_cell_for__class__(member.fdel, cls, newcls)):
                 break
 
+    # gh-135228: Make sure the original class can be garbage collected.
+    # Bypass mapping proxy to allow __dict__ to be removed
+    old_cls_dict = cls.__dict__ | _deproxier
+    old_cls_dict.pop('__dict__', None)
+    if "__weakref__" in cls.__dict__:
+        del cls.__weakref__
+
     return newcls
 
 
@@ -1732,3 +1739,11 @@ def _replace(self, /, **changes):
     # changes that aren't fields, this will correctly raise a
     # TypeError.
     return self.__class__(**changes)
+
+
+# Hack to the get the underlying dict out of a mappingproxy
+# Use it with: cls.__dict__ | _deproxier
+class _Deproxier:
+    def __ror__(self, other):
+        return other
+_deproxier = _Deproxier()