Run Flask using python3 not python

There are several ways:

1. Virtualenv lets you create isolated python environments with different versions (this is the way I would recommend)

2. You can put #!/usr/bin/python3 on top of your python file (see here)

3. Or you can start your script with python3 script.py

4. As mentioned in the comments above you can also start your script inside a Docker container that hosts a python3 installation