Fix for operation on closed file in faulthandler teardown

[bnomis]

Closes #11572

remember stderr fileno in config.stash to be restored on faulthandler teardown

[bluetech]

I think this is a good change 👍 .

I think we should simplify a bit by combining fault_handler_original_stderr_fd_key and fault_handler_originally_enabled_key; specifically, scrape fault_handler_originally_enabled_key and use fault_handler_original_stderr_fd_key to see if faulthandler was enabled before or not.

Also agree with @RonnyPfannschmidt that a comment explaining that, in case the faulthandler is already enabled, the faulthandler api doesn't provide a way to get the FD that was previously passed to enable(), so we assume it was the default (stderr).

[bnomis]

Can this PR be merged? Or is there more specific changes needed? Thanks.

[bluetech]

@bnomis There are some unresolved comments above - add a comment and combine the keys (see my comment above), address the coverage issue mentioned by @nicoddemus.

[bnomis]

@bluetech thank you for your comments. I have updated the PR.

[bluetech]

Thanks @bnomis!