input: Write a python function to find the number of days in a given month and year.
  assert days_in_month(2023, 1) == 31
  assert days_in_month(2023, 2) == 28
  assert days_in_month(2020, 2) == 29
  assert days_in_month(2023, 4) == 30
  assert days_in_month(2024, 2) == 29
output: def days_in_month(year, month):
      import calendar
      return calendar.monthrange(year, month)[1]