Update core.py

uncertainties/core.py

@@ -15,7 +15,7 @@
 from __future__ import division  # Many analytical derivatives depend on this
 
 from builtins import str, zip, range, object
-from math import sqrt, isnan, isinf  # Optimization: no attribute look-up
+from math import sqrt, isfinite  # Optimization: no attribute look-up
 
 import copy
 import collections