Add __ne__ built-in function

In Python 3 __ne__ by default delegates to __eq__ and inverts the
result, but in Python 2 they urge you to define __ne__ when you
define __eq__ for it to work properly [1].There are no implied
relationships among the comparison operators. The truth of x==y
does not imply that x!=y is false. Accordingly, when defining __eq__(),
one should also define __ne__() so that the operators will behave
as expected.
[1]https://docs.python.org/2/reference/datamodel.html#object.__ne__

Change-Id: I2eecd9d90a3a6ba09917f27f2f8568a42beb58d1

tests/api/base_api.py

@@ -91,6 +91,9 @@ class DateMatcher(object):
     def __eq__(self, other):
         return other == self.date
 
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
 
 def a_date_matching(date_string):
     return DateMatcher(datetime.strptime(date_string, "%Y-%m-%d %H:%M:%S.%f"))