Remplaced a calendar function not available in Python 3.6

matemat/util/monthdelta.py

@@ -20,7 +20,7 @@ def add_months(d: datetime, months: int) -> datetime:
     days: int = 0
     # Iterate the months between the passed date and the target month
     for i in range(months):
-        days += calendar.monthlen(*nextmonth)
+        days += calendar.monthrange(*nextmonth)[1]
         nextmonth = calendar.nextmonth(*nextmonth)
     # Set the day of month temporarily to 1, then add the day offset to reach the 1st of the target month
     newdate: datetime = d.replace(day=1) + timedelta(days=days)