input: Write a python function to decode a base64 encoded string.
  assert decode_string('aGVsbG8=') == 'hello'
  assert decode_string('d29ybGQ=') == 'world'
  assert decode_string('UHl0aG9u') == 'Python'
output: def decode_string(encoded):
      import base64
      decoded_bytes = base64.b64decode(encoded)
      return decoded_bytes.decode('utf-8')