Message 204707 - Python tracker

AFAICT, there's no link (FWIW I wrote the patch for #10517, then the fix
for threads created outside of Python calling into a subinterpreter -
issue #13156).
Actually, this "fix" would have avoided the regression in issue #13156.
But since it's tricky, I'd really like if Graham could test it (his module
does a lot of nasty things that could not show up in our test suite).