input: Write a function named is_palindrome that takes a string s as input and returns True if the string s is a palindrome and False if it isn't.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("level")==True
  assert is_palindrome("")==True
  assert is_palindrome("a")==True
output: def is_palindrome(s):
      return s == s[::-1]