Newly added test_threaded_weak_valued_pop() triggers ignored KeyError exceptions (on all branches):

$ python3.7 -m test -v test_weakref
...
0:00:00 [1/1] test_weakref
...
test_make_weak_keyed_dict_from_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_keyed_dict_from_weak_keyed_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_keyed_dict_repr (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_from_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_from_weak_valued_dict (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_misc (test.test_weakref.MappingTestCase) ... ok
test_make_weak_valued_dict_repr (test.test_weakref.MappingTestCase) ... ok
test_threaded_weak_valued_pop (test.test_weakref.MappingTestCase) ... Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
Exception ignored in: <function WeakValueDictionary.__init__.<locals>.remove at 0x7fb20612bb70>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/weakref.py", line 114, in remove
    del self.data[wr.key]
KeyError: (10,)
ok
test_threaded_weak_valued_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_bad_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_cascading_deletes (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_popitem (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_dict_update (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_iters (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_len_cycles (test.test_weakref.MappingTestCase) ... ok
test_weak_keyed_len_race (test.test_weakref.MappingTestCase) ... ok
test_weak_keys (test.test_weakref.MappingTestCase) ... ok
test_weak_keys_destroy_while_iterating (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_delitem (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_popitem (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_setdefault (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_dict_update (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_iters (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_len_cycles (test.test_weakref.MappingTestCase) ... ok
test_weak_valued_len_race (test.test_weakref.MappingTestCase) ... ok
test_weak_values (test.test_weakref.MappingTestCase) ... ok
test_weak_values_destroy_while_iterating (test.test_weakref.MappingTestCase) ... ok
...

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Ran 116 tests in 3.641s
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