input: Write a python function to check if two strings are permutations of each other.
  assert is_permutation("abc", "cba") == True
  assert is_permutation("hello", "lloeh") == True
  assert is_permutation("test", "sett") == True
  assert is_permutation("python", "nothpy") == True
  assert is_permutation("hello", "world") == False
  assert is_permutation("abc", "ab") == False
output: def is_permutation(s1, s2):
      if len(s1) != len(s2):
          return False
      return sorted(s1) == sorted(s2)