Oh. I identified the root issue: Tools/scripts/2to3 is installed as Tools/scripts/2to3.py, a ".py" suffix is added. This script is tested on all non-Windows and skipped on Windows when run from source code, but test_sundry() imports 2to3.py which causes the test to fail, since the script expects an argument on the command line and fails with SystemExit which is not catched by test_sundry().

I wrote PR 8406 to skip 2to3.py. My PR also removes imported modules to remove side effects of the test.

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Python 2.7 is not affected: test_tools is skipped on installed Python:

vstinner@WIN C:\>py -2.7 -m test test_tools                                     
...
test_tools skipped -- test irrelevant for an installed Python
...
Tests result: SUCCESS