Attempt to fix test failure with generate_tokens

It looks like the try/except comments were wrong and generate_tokens is available on Py3

IPython/core/ultratb.py

@@ -101,7 +101,7 b' import time'
 import tokenize
 import traceback
 
-generate_tokens = tokenize.tokenize
+from tokenize import generate_tokens
 
 # For purposes of monkeypatching inspect to fix a bug in it.
 from inspect import getsourcefile, getfile, getmodule, \