Currently `_xxsubinterpreters.run_string()` fails if the interpreter has other threads running, e.g. subthreads created by a previous `run_string()` call. This should be fixed to allow other threads to be running. <!-- gh-linked-prs --> ### Linked PRs * gh-109921 * gh-110707 * gh-112500 * gh-117049 * gh-117140 <!-- /gh-linked-prs -->