Add assert_not_in method for Python2.6

IPython/testing/nose_assert_methods.py

@@ -1,11 +1,17 b''
 """Add some assert methods to nose.tools. These were added in Python 2.7/3.1, so
 once we stop testing on Python 2.6, this file can be removed.
 """
 
 import nose.tools as nt
 
 def assert_in(item, collection):
     assert item in collection, '%r not in %r' % (item, collection)
 
 if not hasattr(nt, 'assert_in'):
     nt.assert_in = assert_in
+
+def assert_not_in(item, collection):
+    assert item not in collection, '%r in %r' % (item, collection)
+
+if not hasattr(nt, 'assert_not_in'):
+    nt.assert_not_in = assert_not_in