Skip test that builtin signature can't be inspected

Builtins can now provide a signature, and sum() does so as of Python
3.5. I thought this is related to PEP 436, but that still appears to be
in draft status, so I'm not sure.

IPython/core/tests/test_oinspect.py

@@ -203,6 +203,7 b' def test_calltip_function2():'
     check_calltip(g, 'g', 'g(y, z=3, *a, **kw)', '<no docstring>')
 
 
+@skipif(sys.version_info >= (3, 5))
 def test_calltip_builtin():
     check_calltip(sum, 'sum', None, sum.__doc__)