I got the following output when run tests in the huntrleaks mode.

$ ./python -We -m test -R 3:3 -x test_builtin -x test_urlparse
...
1:24:54 load avg: 2.48 [225/414] test_multiprocessing_fork
beginning 6 repetitions
123456
..python: Python/pystate.c:589: PyThreadState_Clear: Assertion `tstate->exc_info->previous_item == NULL' failed.
Fatal Python error: Aborted

Current thread 0x00007fdd3e07e700 (most recent call first):
  File "/home/serhiy/py/cpython/Lib/multiprocessing/popen_fork.py", line 70 in _launch
  File "/home/serhiy/py/cpython/Lib/multiprocessing/popen_fork.py", line 20 in __init__
  File "/home/serhiy/py/cpython/Lib/multiprocessing/context.py", line 277 in _Popen
  File "/home/serhiy/py/cpython/Lib/multiprocessing/process.py", line 112 in start
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 241 in _repopulate_pool
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 248 in _maintain_pool
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 412 in _handle_workers
  File "/home/serhiy/py/cpython/Lib/threading.py", line 865 in run
  File "/home/serhiy/py/cpython/Lib/threading.py", line 917 in _bootstrap_inner
  File "/home/serhiy/py/cpython/Lib/threading.py", line 885 in _bootstrap
..python: Python/pystate.c:589: PyThreadState_Clear: Assertion `tstate->exc_info->previous_item == NULL' failed.
Fatal Python error: Aborted

Current thread 0x00007fdd6cb6c700 (most recent call first):
  File "/home/serhiy/py/cpython/Lib/multiprocessing/popen_fork.py", line 70 in _launch
  File "/home/serhiy/py/cpython/Lib/multiprocessing/popen_fork.py", line 20 in __init__
  File "/home/serhiy/py/cpython/Lib/multiprocessing/context.py", line 277 in _Popen
  File "/home/serhiy/py/cpython/Lib/multiprocessing/process.py", line 112 in start
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 241 in _repopulate_pool
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 248 in _maintain_pool
  File "/home/serhiy/py/cpython/Lib/multiprocessing/pool.py", line 412 in _handle_workers
  File "/home/serhiy/py/cpython/Lib/threading.py", line 865 in run
  File "/home/serhiy/py/cpython/Lib/threading.py", line 917 in _bootstrap_inner
  File "/home/serhiy/py/cpython/Lib/threading.py", line 885 in _bootstrap
..
1:34:07 load avg: 1.01 [226/414] test_multiprocessing_forkserver -- test_multiprocessing_fork passed in 9 min 13 sec
...

It happens after 1.5 hours of running tests. Running the test_multiprocessing_fork test only doesn't expose it. Likely there are already opened related issues, but the assertion failure in PyThreadState_Clear is new, it was added in 3.7. It may help to identify the problem.