bpo-41521: Rename blacklist parameter to not_exported (GH-21824)

Rename "blacklist" parameter of test.support.check__all__() to
"not_exported".
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 47e131a..d02d3b3 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -1365,9 +1365,9 @@
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
         extra = {"ThreadError"}
-        blacklist = {'currentThread', 'activeCount'}
+        not_exported = {'currentThread', 'activeCount'}
         support.check__all__(self, threading, ('threading', '_thread'),
-                             extra=extra, blacklist=blacklist)
+                             extra=extra, not_exported=not_exported)
 
 
 class InterruptMainTests(unittest.TestCase):