Thanks for using Bitnami. Our Django 1.11 stack keeps using Python 2.7 series for compatibility with other of our Python applications which don't support Python 3.x versions yet. You can use the Bitnami Django 2.2.6 which includes Python 3.7 and test your application on it (I understand you are using your own Django project on top of our stack).
Apart from that, for the changes required to migrate from Python 2 to Python 3, you can check the official Python documentation linked below
Hope it helps,