[3.13] gh-120065: Increase `collect_in_thread` period to 5 ms. (GH-120068) (#120110)

This matches the default GIL switch interval. It greatly speeds up the
free-threaded build: previously, it spent nearly all its time in
`gc.collect()`.
(cherry picked from commit 4bba1c9e6cfeaf69302b501a4306668613db4b28)

Co-authored-by: Sam Gross <colesbury@gmail.com>