Fix deepreload tests in Python 2.6.

Apparently assert_is_instance and assert_not_is_instance are not available
in Python 2.6.

IPython/lib/tests/test_deepreload.py

@@ -44,9 +44,9 b' def test_deepreload():'
         # Test that A is not reloaded.
         obj = A.Object()
         dreload(B, exclude=['A'])
-        nt.assert_is_instance(obj, A.Object)
+        nt.assert_true(isinstance(obj, A.Object))
 
         # Test that A is reloaded.
         obj = A.Object()
         dreload(B)
-        nt.assert_not_is_instance(obj, A.Object)
+        nt.assert_false(isinstance(obj, A.Object))