Resolve deprecation warning

See [1] for more info.

[1] python/cpython#79893

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

testtools/testcase.py

@@ -269,8 +269,10 @@ def _reset(self):
 
     def __eq__(self, other):
         eq = getattr(unittest.TestCase, "__eq__", None)
-        if eq is not None and not unittest.TestCase.__eq__(self, other):
-            return False
+        if eq is not None:
+            eq_ = unittest.TestCase.__eq__(self, other)
+            if eq_ is NotImplemented or not eq_:
+                return False
         return self.__dict__ == getattr(other, "__dict__", None)
 
     # We need to explicitly set this since we're overriding __eq__