[mypyc] Fix 3.12 issue with pickling of instances with __dict__ (#15574)

This fixes the testPickling test case in
mypyc/test-data/run-classes.test on Python 3.12.

Handle another case that was missed in #15471. On 3.12 we rely on
Py_TPFLAGS_MANAGED_DICT and shouldn't define an explicit `__dict__`
member.

Work on mypyc/mypyc#995.

mypyc/codegen/emitclass.py

@@ -270,7 +270,7 @@ def emit_line() -> None:
     # that isn't what we want.
 
     # XXX: there is no reason for the __weakref__ stuff to be mixed up with __dict__
-    if cl.has_dict:
+    if cl.has_dict and not has_managed_dict(cl, emitter):
         # __dict__ lives right after the struct and __weakref__ lives right after that
         # TODO: They should get members in the struct instead of doing this nonsense.
         weak_offset = f"{base_size} + sizeof(PyObject *)"