Unbound methods are gone in Py3 so there is no easy way to support 'o…

…verridden' function

for unbound methods in Py3 because they look like regular functions and doesn't store reference to a class.
That feature is not used in NLTK own codebase so I removed it from tests. This make 'nltk.internals' tests
("tox -- nltk.internals internals.doctest") pass for all platforms.

nltk/test/internals.doctest

@@ -33,7 +33,7 @@ perform the other method:
     'yum'
     >>> GoodEater1().batch_eat(['steak', 'peas'])
     ['yum', 'yum']
-    
+
     >>> class GoodEater2(EaterI):
     ...     def batch_eat(self, foods):
     ...         return ['yum' for food in foods]
@@ -79,12 +79,12 @@ It's ok to use intermediate abstract classes:
     >>> class GoodEater3(AbstractEater):
     ...     def eat(self, food):
     ...         return 'yum'
-    ...     
+    ...
     >>> GoodEater3().eat('steak')
     'yum'
     >>> GoodEater3().batch_eat(['steak', 'peas'])
     ['yum', 'yum']
-    
+
     >>> class GoodEater4(AbstractEater):
     ...     def batch_eat(self, foods):
     ...         return ['yum' for food in foods]
@@ -122,19 +122,8 @@ Here's some extra tests:
     >>> overridden(D().f)
     True
 
-overridden() can be called on unbound methods as well:
-
-    >>> overridden(A.f)
-    False
-    >>> overridden(B.f)
-    True
-    >>> overridden(C.f)
-    False
-    >>> overridden(D.f)
-    True
-
 It works for classic classes, too:
- 
+
     >>> class A:
     ...     def f(x): pass
     >>> class B(A):
@@ -149,12 +138,3 @@ It works for classic classes, too:
     False
     >>> overridden(D().f)
     True
-    >>> overridden(A.f)
-    False
-    >>> overridden(B.f)
-    True
-    >>> overridden(C.f)
-    False
-    >>> overridden(D.f)
-    True
-