dict insert_split_key() change introduces data-race

[nascheme]

Bug report

Bug description:

GH-152200 fixes a deadlock but it introduces a data-race, resulting in a correctness bug. Demo of bug is linked to other issue. Suggested fix is to revert the original optimization (GH-150490) and also the fix applied to it.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-152238: Revert gh-150490 and gh-152200. #152232