Increase code coverage in Python/structmember.c from 33% to 40%

(by forcing a call to the listmember() function).

Lib/test/test_traceback.py

@@ -85,6 +85,14 @@ def test():
                 os.unlink(os.path.join(testdir, f))
             os.rmdir(testdir)
 
+    def test_members(self):
+        # Covers Python/structmember.c::listmembers()
+        try:
+            1/0
+        except:
+            import sys
+            sys.exc_traceback.__members__
+
 def test_main():
     run_unittest(TracebackCases)