Issue 30813: test_unittest fails when hunting reference leaks

Example of Python 3.5 (same error on master):

haypo@selma$ ./python -m test -R 3:3 -v test_unittest 
(...)
======================================================================
ERROR: test_discover_with_init_module_that_raises_SkipTest_on_import (unittest.test.test_discovery.TestDiscovery)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/haypo/prog/python/3.5/Lib/unittest/test/test_discovery.py", line 572, in test_discover_with_init_module_that_raises_SkipTest_on_import
    pickle.loads(pickle.dumps(suite, proto))
_pickle.PicklingError: Can't pickle <class 'unittest.loader.ModuleSkipped'>: attribute lookup ModuleSkipped on unittest.loader failed

======================================================================
ERROR: test_discover_with_module_that_raises_SkipTest_on_import (unittest.test.test_discovery.TestDiscovery)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/haypo/prog/python/3.5/Lib/unittest/test/test_discovery.py", line 548, in test_discover_with_module_that_raises_SkipTest_on_import
    pickle.loads(pickle.dumps(suite, proto))
_pickle.PicklingError: Can't pickle <class 'unittest.loader.ModuleSkipped'>: attribute lookup ModuleSkipped on unittest.loader failed