Use math.gcd instead of fractions.gcd when possible

fractions.gcd() function has been removed from Python 3.9, it was
deprecated since Python 3.5.

https://docs.python.org/3.9/whatsnew/3.9.html#removed

theano/tensor/nnet/abstract_conv.py

@@ -6,7 +6,10 @@
 import logging
 from six import reraise, integer_types
 import sys
-from fractions import gcd
+try:
+    from math import gcd
+except ImportError:
+    from fractions import gcd
 
 import theano