Resolve "Introduce function to get the last week of a year"

Makes use of the behavior described in https://en.wikipedia.org/wiki/ISO_week_date#Last_week

Closes #6 (closed)

calendarweek/calendarweek.py

@@ -94,6 +94,13 @@ class CalendarWeek:
 
         return weeks
 
+    @classmethod
+    def get_last_week_of_year(cls, year: int) -> CalendarWeek:
+        """Get the last week of a year."""
+
+        last_week = date(year, 12, 28).isocalendar()[1]
+        return cls(week=last_week, year=year)
+
     def __post_init__(self) -> None:
         today = date.today()