Running these 3 tests is enough to reproduce the issue:
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vstinner@DESKTOP-DK7VBIL C:\vstinner\python\master>python -m test test_interpreters test_pdb test_threading  
Running Debug|x64 interpreter...
0:00:00 Run tests sequentially
0:00:00 [1/3] test_interpreters
0:00:04 [2/3] test_pdb
test test_pdb failed -- Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\doctest.py", line 2205, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for test.test_pdb.test_pdb_issue_20766
  File "C:\vstinner\python\master\lib\test\test_pdb.py", line 1270, in test_pdb_issue_20766
(...)
0:00:10 load avg: 0.03 [3/3/1] test_threading -- test_pdb failed
(Pdb) 
-------------------

Moreover, my PR 25674 (bpo-43963) fix these tests.