input: Write a python function to get the ISO week number from a given date string.
  assert get_week_number('2023-01-01')==52
  assert get_week_number('2023-05-15')==20
  assert get_week_number('2023-12-31')==52
output: def get_week_number(date_string):
      from datetime import datetime
      date = datetime.strptime(date_string, '%Y-%m-%d')
      return date.isocalendar()[1]