[frontend/statistics] Fixing missing timezone in datetime key

Using datetime objects as dictionnary keys will change depending on if a timezone was given. If a timezone is given the key will have an offset in addition to the date and time. In our case, adding the utc timezone to the c key solves the problem.

inginious/frontend/pages/course_admin/statistics.py

@@ -95,7 +95,7 @@ def _graph_stats(self, daterange, filter, limit):
             cur += increment
 
         for entry in stats_graph:
-            c = datetime(entry["_id"]["year"], entry["_id"]["month"], entry["_id"]["day"], 0 if method == "day" else entry["_id"]["hour"])
+            c = datetime(entry["_id"]["year"], entry["_id"]["month"], entry["_id"]["day"], 0 if method == "day" else entry["_id"]["hour"], tzinfo=timezone.utc)
             all_submissions[c] += entry["submissions"]
             valid_submissions[c] += entry["validSubmissions"]