gh-120065: Increase collect_in_thread period to 5 ms. (#120068)

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()`.

Lib/test/test_weakref.py

@@ -82,7 +82,7 @@ def callback(self, ref):
 
 
 @contextlib.contextmanager
-def collect_in_thread(period=0.0001):
+def collect_in_thread(period=0.005):
     """
     Ensure GC collections happen in a different thread, at a high frequency.
     """