input: Write a python function to check if a given year is a leap year.
  assert is_leap_year(2000) == True
  assert is_leap_year(1900) == False
  assert is_leap_year(2004) == True
  assert is_leap_year(2001) == False
output: def is_leap_year(year):
      if year % 4 == 0:
          if year % 100 == 0:
              if year % 400 == 0:
                  return True
              else:
                  return False
          else:
              return True
      else:
          return False