BUG: Fix test_void_dtype_equality_failures for Python 3.

The NotArray test class needs to define a `__ne__` method, otherwise the
inherited Python 3 method will call `__eq__`, resulting in two rather
than one DeprecationWarning.

numpy/core/tests/test_deprecations.py

@@ -75,6 +75,7 @@ def assert_deprecated(self, function, num=1, ignore_others=False,
             function(*args, **kwargs)
         except (Exception if function_fails else tuple()):
             pass
+
         # just in case, clear the registry
         num_found = 0
         for warning in self.log:
@@ -447,6 +448,10 @@ class NotArray(object):
             def __array__(self):
                 raise TypeError
 
+            # Needed so Python 3 does not raise DeprecationWarning twice.
+            def __ne__(self, other):
+                return NotImplemented
+
         self.assert_deprecated(lambda: np.arange(2) == NotArray())
         self.assert_deprecated(lambda: np.arange(2) != NotArray())