input: Write a function to check whether a given year is a leap year or not.
  assert is_leap_year(2020) == True
  assert is_leap_year(1900) == False
  assert is_leap_year(2000) == True
  assert is_leap_year(2021) == False
  assert is_leap_year(2400) == True
output: def is_leap_year(year):
      if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
          return True
      else:
          return False