Seen on a buildbot:

test test_multiprocessing crashed -- Traceback (most recent call last):
  File "/export/home/buildbot/32bits/3.x.cea-indiana-x86/build/Lib/test/regrtest.py", line 1229, in runtest_inner
    the_package = __import__(abstest, globals(), locals(), [])
  File "<frozen importlib._bootstrap>", line 1030, in _find_and_load
  File "<frozen importlib._bootstrap>", line 611, in load_module
  File "<frozen importlib._bootstrap>", line 271, in module_for_loader_wrapper
  File "<frozen importlib._bootstrap>", line 499, in _load_module
  File "/export/home/buildbot/32bits/3.x.cea-indiana-x86/build/Lib/test/test_multiprocessing.py", line 2430, in <module>
    testcases_processes = create_test_cases(ProcessesMixin, type='processes')
  File "/export/home/buildbot/32bits/3.x.cea-indiana-x86/build/Lib/test/test_multiprocessing.py", line 2409, in create_test_cases
    class Temp(base, unittest.TestCase, Mixin):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases


Here is a patch.