Fix Windows.

Author: ericsnowcurrently

Lib/threading.py

@@ -50,7 +50,10 @@
     _CRLock = None
 TIMEOUT_MAX = _thread.TIMEOUT_MAX
 _wait_for_threads_fini = _thread._wait_for_threads_fini
-_internal_after_fork = _thread._after_fork
+try:
+    _internal_after_fork = _thread._after_fork
+except AttributeError:
+    _internal_after_fork = None
 del _thread
 
 
@@ -1683,5 +1686,6 @@ def _after_fork():
 
 
 if hasattr(_os, "register_at_fork"):
-    _os.register_at_fork(after_in_child=_internal_after_fork)
+    if _internal_after_fork is not None:
+        _os.register_at_fork(after_in_child=_internal_after_fork)
     _os.register_at_fork(after_in_child=_after_fork)