input: Write a python function to determine the number of days in a given year.
  assert days_in_year(2000) == 366
  assert days_in_year(1900) == 365
  assert days_in_year(2020) == 366
  assert days_in_year(2021) == 365
output: def days_in_year(year):
      if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
          return 366
      else:
          return 365