English ordinal suffix for the day of the month, 2 characters; i.e. 'st', 'nd', 'rd' or 'th

def S(self):
    "English ordinal suffix for the day of the month, 2 characters; i.e. 'st', 'nd', 'rd' or 'th'"
    if self.data.day in (11, 12, 13): # Special case
      return u'th'
    last = self.data.day % 10
    if last == 1:
      return u'st'
    if last == 2:
      return u'nd'
    if last == 3:
      return u'rd'
    return u'th'