I have been playing with the reproducer and I am a bit confused: The reproducer crashes in the same way even after reverting PR 19414 so it does not seem related to it. This is what I get:

>>> import reproducer
>>> reproducer.Modules/gcmodule.c:114: gc_decref: Assertion "gc_get_refs(g) > 0" failed: refcount is too small
Enable tracemalloc to get the memory block allocation traceback

object address  : 0x55cf429e4080
object refcount : 4
object type     : 0x55cf418e9c60
object type name: type
object repr     : <class 'reproducer.Base'>

Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized

Current thread 0x00007fc65d2f8740 (most recent call first):
  File "<frozen importlib._bootstrap>", line 917 in _find_spec
  File "<frozen importlib._bootstrap>", line 982 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1007 in _find_and_load
  File "/home/pablogsal/github/python/master/Lib/re.py", line 124 in <module>
  File "<frozen importlib._bootstrap>", line 228 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 790 in exec_module
  File "<frozen importlib._bootstrap>", line 680 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 986 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1007 in _find_and_load
  File "/home/pablogsal/github/python/master/Lib/rlcompleter.py", line 142 in attr_matches
  File "/home/pablogsal/github/python/master/Lib/rlcompleter.py", line 89 in complete
[1]    162397 abort (core dumped)  ./python

This seems to indicate that that reproducer is already implementing incorrectly tp_traverse.