Trivial cleanups following bpo-31370 (#3649)

* Trivial cleanups following bpo-31370

* Also cleanup the "importlib._bootstrap_external" module
diff --git a/Lib/test/test_threadsignals.py b/Lib/test/test_threadsignals.py
index f93dd77..67a1c58 100644
--- a/Lib/test/test_threadsignals.py
+++ b/Lib/test/test_threadsignals.py
@@ -5,7 +5,7 @@
 import os
 import sys
 from test import support
-thread = support.import_module('_thread')
+import _thread as thread
 import time
 
 if (sys.platform[:3] == 'win'):