Replace base64.decodestring() with base64.decodebytes()
Replace base64.decodestring() with base64.decodebytes()

base64.decodestring() was an alias for base64.decodebytes() that was
deprecated since Python 3.1, and has been removed in Python 3.9.

hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock