exceptions.ErrorDetail: Handle NotImplemented correctly in __ne__ (#8538)

PR #7531 resolved issue #7433 by updating `ErrorDetails.__eq__` to correctly
handle the `NotImplemented` case. However, Python 3.9 continues to issue the
following warning:

    DeprecationWarning: NotImplemented should not be used in a boolean context

This is because `__ne__` still doesn't handle the `NotImplemented` case
correctly. In order to avoid this warning, this commit makes the same change
for `__ne__` as previously made for `__eq__`.

Author: allanlewis

rest_framework/exceptions.py

@@ -81,7 +81,10 @@ def __eq__(self, other):
             return r
 
     def __ne__(self, other):
-        return not self.__eq__(other)
+        r = self.__eq__(other)
+        if r is NotImplemented:
+            return NotImplemented
+        return not r
 
     def __repr__(self):
         return 'ErrorDetail(string=%r, code=%r)' % (