testCMSSWVersions test broken in python 3.9

[khurtado]

Impact of the bug
Running the following unit tests breaks due to a python 3.9 threading issue:

testCMSSWVersions

Describe the bug
When running the testCMSSWVersions unit test, we get an error outside our code, related to python 3.9:

 File "/usr/lib64/python3.9/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/lib64/python3.9/unittest/case.py", line 588, in run
    self._callSetUp()
  File "/usr/lib64/python3.9/unittest/case.py", line 547, in _callSetUp
    self.setUp()
  File "/home/cmsbld/WMCore/test/python/WMCore_t/ReqMgr_t/Service_t/Auxiliary_t.py", line 30, in setUp
    RESTBaseUnitTestWithDBBackend.setUp(self)
  File "/home/cmsbld/WMCore/src/python/WMQuality/REST/RESTBaseUnitTestWithDBBackend.py", line 61, in setUp
    CherrypyTestInit.start(self.server)
  File "/home/cmsbld/WMCore/src/python/WMQuality/CherrypyTestInit.py", line 11, in start
    server.start(blocking=False)
  File "/home/cmsbld/WMCore/src/python/WMQuality/REST/ServerSetup.py", line 55, in start
    cherrypy.engine.start()
  File "/usr/local/lib/python3.9/site-packages/cherrypy/process/wspbus.py", line 278, in start
    self.exit()
  File "/usr/local/lib/python3.9/site-packages/cherrypy/process/wspbus.py", line 310, in exit
    os._exit(EX_SOFTWARE)
  File "/home/cmsbld/WMCore/setup_test.py", line 109, in trapExit
    raise SystemExit("os._exit() was called in a child thread. " + \

How to reproduce it
Run unit test above

Expected behavior
We should not get a raised error from os._exit()

Additional context and error message
Related issue: #11978