input: Write a python function to decode a base64 encoded string.
  assert decode_base64('SGVsbG8gd29ybGQ=') == 'Hello world'
  assert decode_base64('UHl0aG9uIGlzIGF3ZXNvbWU=') == 'Python is awesome'
  assert decode_base64('VGhpcyBpcyBhIHRlc3Qgc3RyaW5nLg==') == 'This is a test string.'
output: def decode_base64(encoded_str):
      import base64
      decoded_bytes = base64.b64decode(encoded_str)
      decoded_str = decoded_bytes.decode('utf-8')
      return decoded_str