fix: set Py_TPFLAGS_MANAGED_WEAKREF flag on python>=3.12

Author: BobTheBuidler

mypyc/codegen/emitclass.py

@@ -323,6 +323,7 @@ def emit_line() -> None:
             "};",
         )
         if emitter.capi_version < (3, 12):
+            # versions >= 3.12 do not define tp_weaklistoffset, but set Py_TPFLAGS_MANAGED_WEAKREF flag instead
             fields["tp_weaklistoffset"] = base_size
     else:
         fields["tp_basicsize"] = base_size
@@ -384,6 +385,9 @@ def emit_line() -> None:
             fields["tp_call"] = "PyVectorcall_Call"
     if has_managed_dict(cl, emitter):
         flags.append("Py_TPFLAGS_MANAGED_DICT")
+    if cl.supports_weakref and emitter.capi_version >= (3, 12):
+        flags.append("Py_TPFLAGS_MANAGED_WEAKREF")
+    
     fields["tp_flags"] = " | ".join(flags)
 
     fields["tp_doc"] = native_class_doc_initializer(cl)