input: Write a function to reverse a string using recursion.
  assert reverse_string("Hello") == "olleH"
  assert reverse_string("world") == "dlrow"
  assert reverse_string("Python") == "nohtyP"
output: def reverse_string(s):
      if len(s) == 0:
          return ""
      else:
          return s[-1] + reverse_string(s[:-1])