Use math.gcd

fractions.gcd is deprecated from Python 3.5 and it's been removed in Python 3.9, so using fractions.gcd
the activity fails to start with "ImportError: cannot import name 'gcd' from 'fractions'" on F33 SoaS as it uses Python 3.9

Signed-off-by: Ibiam Chihurumnaya <ibiamchihurumnaya@gmail.com>

historietaactivity.py

@@ -8,7 +8,7 @@
 import logging
 from gettext import gettext as _
 import time
-from fractions import gcd
+from math import gcd
 
 import gi
 gi.require_version('Gst', '1.0')