Make `_struct` module thread-safe in `--disable-gil` builds

[colesbury]

Feature or enhancement

The _struct module has a few small issues:

• The use of PyDict_GetItemWithError returns a borrowed reference (should use PyDict_GetItemRef)
• The state->cache is lazily created; we should instead create it during _structmodule_exec
• We want state->cache to be an immutable reference to a mutable dict. (The dict will be thread-safe.) Use PyDict_Clear to empty the dict instead of clearing the reference.

See the commit from the nogil-3.12 fork for context: colesbury/nogil-3.12@ada9b73feb. Note that in CPython the relevant function is PyDict_GetItemRef not PyDict_FetchItemWithError.

Linked PRs

• gh-112062: Make _struct module thread-safe in --disable-gil builds #112094

[chgnrdv]

I'll submit a PR :)