Following up, I was able to recreate under 3.8 using pdb in powershell, so it's not just in IDLE.

Traceback (most recent call last):
  File "N:\projects\cpython\lib\pdb.py", line 1697, in main
    pdb._runscript(mainpyfile)
  File "N:\projects\cpython\lib\pdb.py", line 1566, in _runscript
    self.run(statement)
  File "N:\projects\cpython\lib\bdb.py", line 586, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "c:\users\onedrive\desktop\step.py", line 1, in <module>
    import unittest
  File "<frozen importlib._bootstrap>", line 980, in _find_and_load
  File "<frozen importlib._bootstrap>", line 149, in __enter__
  File "<frozen importlib._bootstrap>", line 101, in acquire
KeyError: 14104