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

bpo-36565: Fix libregrtest for Python without builtin _abc (GH-12733) · python/cpython@79b5d29

Original file line number	Diff line number	Diff line change
`@@ -8,9 +8,13 @@`
`8`	`8`	`try:`
`9`	`9`	`from _abc import _get_dump`
`10`	`10`	`except ImportError:`
	`11`	`+import weakref`
	`12`	`+`
`11`	`13`	`def _get_dump(cls):`
`12`		`-# For legacy Python version`
`13`		`-return (cls._abc_registry, cls._abc_cache,`
	`14`	`+# Reimplement _get_dump() for pure-Python implementation of`
	`15`	`+# the abc module (Lib/_py_abc.py)`
	`16`	`+registry_weakrefs = set(weakref.ref(obj) for obj in cls._abc_registry)`
	`17`	`+return (registry_weakrefs, cls._abc_cache,`
`14`	`18`	`cls._abc_negative_cache, cls._abc_negative_cache_version)`
`15`	`19`
`16`	`20`