input: Write a python function to decode a base64 encoded string.
  assert decode_base64('aGVsbG8=') == 'hello'
  assert decode_base64('UHl0aG9u') == 'Python'
  assert decode_base64('VGhpcyBpcyBhIHRlc3Q=') == 'This is a test'
output: import base64
  
  def decode_base64(encoded_str):
      decoded_bytes = base64.b64decode(encoded_str)
      return decoded_bytes.decode('utf-8')