[3.13] gh-127553: Remove outdated TODO comment in _pydatetime (GH-127564) (#128500)

(cherry picked from commit e8b6b39)

Co-authored-by: Beomsoo Kim <beoms424@gmail.com>

Author: miss-islington

Lib/_pydatetime.py

@@ -2306,7 +2306,6 @@ def __reduce__(self):
 
 def _isoweek1monday(year):
     # Helper to calculate the day number of the Monday starting week 1
-    # XXX This could be done more efficiently
     THURSDAY = 3
     firstday = _ymd2ord(year, 1, 1)
     firstweekday = (firstday + 6) % 7  # See weekday() above