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_mailbox.py b/Lib/test/test_mailbox.py
index 346c9f1..5924f24 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -2296,8 +2296,8 @@
 
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
-        blacklist = {"linesep", "fcntl"}
-        support.check__all__(self, mailbox, blacklist=blacklist)
+        support.check__all__(self, mailbox,
+                             not_exported={"linesep", "fcntl"})
 
 
 def test_main():