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Extract of the crashing test:
 def test_cmp(self):
 ...
 class Exc(Exception):
 pass
 class BadCmp(object):
 def __eq__(self, other):
 raise Exc
 s1 = slice(BadCmp())
 s2 = slice(BadCmp())
 self.assertEqual(s1, s1)
 self.assertRaises(Exc, lambda: s1 == s2) # <==== CRASH HERE
 ...
The latest change in test_slice.py has been made in 2016: 3 years ago.
Latest changes in Objects/sliceobject.c were refactoring related to header files (november 2018). The latest "significant" change was done in April 2018: "bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS".
I bet more on a recent change which is not directly related to slices... like the implementation of the PEP 572? It would help to know when the crash started to occur exactly (date + time).