bpo-36565: Fix libregrtest for Python without builtin _abc (GH-12733)

Fix reference hunting (``python3 -m test -R 3:3``) when Python has no
built-in abc module: fix _get_dump() reimplementation of libregrtest.

Lib/test/libregrtest/refleak.py

@@ -8,9 +8,13 @@ from test import support
 try:
     from _abc import _get_dump
 except ImportError:
+    import weakref
+
     def _get_dump(cls):
-        # For legacy Python version
-        return (cls._abc_registry, cls._abc_cache,
+        # Reimplement _get_dump() for pure-Python implementation of
+        # the abc module (Lib/_py_abc.py)
+        registry_weakrefs = set(weakref.ref(obj) for obj in cls._abc_registry)
+        return (registry_weakrefs, cls._abc_cache,
                 cls._abc_negative_cache, cls._abc_negative_cache_version)
 
 

Misc/NEWS.d/next/Tests/2019-04-08-19-01-21.bpo-36565.2bxgtU.rst

+Fix reference hunting (``python3 -m test -R 3:3``) when Python has no
+built-in abc module.