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Serhiy, it sure is. I'll note that the issue is open. Moreover, reading through the history, Guido says it's a bug (msg160418), and there was agreement that it should be fixed. It's unclear why it was dropped.
Can you help me get this in? I'm not sure the correct protocol, since this is my first contrib.
Note that if this is not a bug, then there are probably bugs anywhere issubclass(...) is used to emulate exception handling. For example, unittest's self.assertRaises is inconsistent with the current behavior:
 from abc import ABC
 import unittest
 
 class TestExceptionABC(unittest.TestCase):
 
 def test_exception(self):
 class A(Exception, ABC):
 pass
 class B(Exception):
 pass
 A.register(B)
 with self.assertRaises(A, msg="Wrong exception raised"):
 raise B
Test passes in python3, but not in python2.