gh-100228: Warn from os.fork() if other threads exist. (#100229)

Not comprehensive, best effort warning. There are cases when threads exist on some platforms that this code cannot detect. macOS when API permissions allow and Linux with a readable /proc procfs present are the currently supported cases where a warning should show up reliably.

Starting with a DeprecationWarning for now, it is less disruptive than something like RuntimeWarning and most likely to only be seen in people's CI tests - a good place to start with this messaging.

Lib/threading.py

@@ -1490,6 +1490,8 @@ def active_count():
     enumerate().
 
     """
+    # NOTE: if the logic in here ever changes, update Modules/posixmodule.c
+    # warn_about_fork_with_threads() to match.
     with _active_limbo_lock:
         return len(_active) + len(_limbo)