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(2016) == True
  assert is_leap_year(2019) == False
  assert is_leap_year(2024) == True
output: def is_leap_year(year):
      if year % 4 != 0:
          return False
      elif year % 100 != 0:
          return True
      elif year % 400 != 0:
          return False
      else:
          return True