don't use f_locals for foreign async generators

[graingert]

Fixes https://github.com/python-trio/trio/issues/2454

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.63%. Comparing base (9d6e134) to head (8fdaaf5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3112 +/- ## ======================================= Coverage 99.62% 99.63% ======================================= Files 122 122 Lines 18340 18380 +40 Branches 1222 1223 +1 ======================================= + Hits 18272 18312 +40 Misses 47 47 Partials 21 21 ``` | [Files with missing lines](https://app.codecov.io/gh/python-trio/trio/pull/3112?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_core/\_asyncgens.py](https://app.codecov.io/gh/python-trio/trio/pull/3112?src=pr&el=tree&filepath=src%2Ftrio%2F_core%2F_asyncgens.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NvcmUvX2FzeW5jZ2Vucy5weQ==) | `100.00% <100.00%> (ø)` | | | [src/trio/\_core/\_tests/test\_guest\_mode.py](https://app.codecov.io/gh/python-trio/trio/pull/3112?src=pr&el=tree&filepath=src%2Ftrio%2F_core%2F_tests%2Ftest_guest_mode.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NvcmUvX3Rlc3RzL3Rlc3RfZ3Vlc3RfbW9kZS5weQ==) | `100.00% <100.00%> (ø)` | |

[A5rocks]

Oh the pypy cache failures are back :(

I tried deleting the relevant caches from the GH actions cache page last time this happened but I guess they came back. That still works to fix it temporarily though. (Or they get fixed after a few hours for some reason)