Issue #27106: Add test for configparser.__all__

Patch by Jacek KoƂodziej. The Error class is deliberately omitted because it
is a generic name and of limited use.
diff --git a/Lib/test/test_configparser.py b/Lib/test/test_configparser.py
index 71a8f3f..2b08198 100644
--- a/Lib/test/test_configparser.py
+++ b/Lib/test/test_configparser.py
@@ -2052,5 +2052,11 @@
             self.assertEqual(cfg['two'].getlen('one'), 5)
 
 
+class MiscTestCase(unittest.TestCase):
+    def test__all__(self):
+        blacklist = {"Error"}
+        support.check__all__(self, configparser, blacklist=blacklist)
+
+
 if __name__ == '__main__':
     unittest.main()