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: Ic7d9b80b2d24f2e10e361fc22cc3439f7b8213ae

congress/datalog/unify.py

@@ -63,6 +63,9 @@ class BiUnifier(object):
         def __eq__(self, other):
             return self.value == other.value and self.unifer == other.unifier
 
+        def __ne__(self, other):
+            return not self.__eq__(other)
+
         def __repr__(self):
             return "Value(value={}, unifier={})".format(
                 repr(self.value), repr(self.unifier))
@@ -79,6 +82,9 @@ class BiUnifier(object):
         def __eq__(self, other):
             return self.var == other.var and self.unifier == other.unifier
 
+        def __ne__(self, other):
+            return not self.__eq__(other)
+
     def __init__(self, dictionary=None):
         # each value is a Value
         self.contents = {}
@@ -165,6 +171,9 @@ class BiUnifier(object):
     def __eq__(self, other):
         return self.contents == other.contents
 
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
 
 def binding_str(binding):
     """Handles string conversion of either dictionary or Unifier."""