Fixing a re-entrancy bug in importlib._bootstrap

I recently encountered the error case described in importlib lock race issue in deadlock handling code · Issue #91351 · python/cpython · GitHub (reported in April). After spending several weeks understanding the problem I have just submitted a PR that fixes at least one and probably two different ways an import can go wrong in the face of re-entrancy.

The PR is at gh-91351: Support re-entrancy in importlib/ by exarkun · Pull Request #94342 · python/cpython · GitHub and I would appreciate any help moving it closer to being merged.