ENH: test for af9cfeb5 fix of weakref objects

tests/run/weakfail.pyx

+import gc
+import weakref
+
+foo_dict = weakref.WeakValueDictionary()
+
+cdef class Foo:
+    cdef object __weakref__
+
+def test_weakref(key):
+    """
+    Test af9cfeb5f94d9cd4f2989fc8e111c33208494ba4 fix.
+    Originally running it using debug build of python lead to::
+
+      visit_decref: Assertion `gc->gc.gc_refs != 0' failed
+
+    >>> _ = gc.collect()
+    >>> _ = test_weakref(48)
+    >>> _ = gc.collect()
+    """
+    obj = Foo()
+    foo_dict[key] = obj
+    return obj
+