remove deprecated API

Author: rscohn2

src/github3/repos/stats.py

@@ -20,7 +20,7 @@ def alternate_week(week):
     :rtype:
         dict
     """
-    start_of_week = datetime.datetime.utcfromtimestamp(int(week["w"]))
+    start_of_week = datetime.datetime.fromtimestamp(int(week["w"]), datetime.UTC)
     return {
         "start of week": start_of_week.replace(tzinfo=dateutil.tz.tzutc()),
         "additions": week["a"],