(Brett, the question is about import.)

The problem is the mode call in clock.py, which I will move today as part of 21882.

I am sorely puzzled that the patch in #21823 could have changed the effect of mode(). There are only two changes, only one of which could be relevant.

1.
 from turtle import *
+from turtle import Terminator  # not in __all__
 mode()

My understanding is that the newly added second import should just add a reference to Terminator in the existing module. It is a standard Exception subclass: from turtle.py, "class Terminator(Exception): pass".  I could and will add 'Terminator' to __all__ instead, but it seems to me that the added statement *should* be innocuous.

What am I missing? Does __all__ change the import machinery in the calls to frozen importlib.bootstrap or do these call always happen behind the scene with any import?  From the most recent first traceback:
...
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/turtledemo/clock.py", line 17 in <module>
  File "<frozen importlib._bootstrap>", line 321 in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1420 in exec_module
  File "<frozen importlib._bootstrap>", line 1149 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 2175 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 2186 in _find_and_load
  File "<string>", line 1 in <module>
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/test___all__.py", line 23 in check_all
...

2. Added a try:except: within a function, not called on import, to catch previously uncaught Terminator exception.