BUG: Handle all possible types of Security object __richcmp__ args

A cython __richcmp__ function isn't allowed to assume that its first
argument is the same as the type of the class to which it belongs, so
our code needs to account for either of its two arguments being of the
wrong type.

Furthermore, the correct way for __richcmp__ to handle when it doesn't
know how to do a comparison is to return NotImplemented.

tests/test_security_object.py

@@ -27,3 +27,7 @@ class TestSecurityRichCmp(TestCase):
         self.assertFalse(Security(3) > Security(4))
         self.assertFalse(Security(4) > Security(4))
         self.assertTrue(Security(5) > Security(4))
+
+    def test_type_mismatch(self):
+        self.assertIsNotNone(Security(3) < 'a')
+        self.assertIsNotNone('a' < Security(3))