[3.9] bpo-39934: Account for control blocks in 'except' in compiler. (GH-22395)

[iritkatriel]

Pushes an artificial control "block" around the except part of a try-except.
This ensures that the computed depth in the compiler matches the actual depth in the runtime.
(cherry picked from commit 02d126a )

https://bugs.python.org/issue39934

[iritkatriel]

I made this PR manually because there were significant merge conflicts. Please check I got it right.

[Fidget-Spinner]

LGTM, though I think you need to change your title to [3.9] bpo-39934: Account for control blocks in 'except' in compiler. (GH-22395) . And the first message should be:

Pushes an artificial control "block" around the `except` part of a try-except.
This ensures that the computed depth in the compiler matches the actual depth in the runtime.
(cherry picked from commit 02d126aa09d96d03dcf9c5b51c858ce5ef386601 )

This will allow the bots to register it as a backport :). For more info, you can check the devguide section on backporting.

One potential concert may be that ADDOP_JREL was changed to ADDOP_JUMP in #21714. Although no behavior should have changed, I hope this doesn't cause problems in the future.

[markshannon]

Thanks for doing this.
I wasn't going to bother with this because it's an obscure bug, it's fixed in 3.10 and I'm lazy 🙂

The changes look good to me.
I presume the NEWS item will need backporting as well.

[iritkatriel]

I've added the news file.

I took this up to familiarize myself with the process. If you prefer to to backport we can just close this and the bpo issue. Up to you.

[markshannon]

This is good. I don't need to do it myself.