Run Flask using python3 not python
What I did was:

which flask, to find flask binary executable path (in my case /home/myuser/.local/bin/flask
edit /home/myuser/.local/bin/flask, changing the first line from #!/usr/bin/python to #!/usr/bin/python3

In summary, making flask use python3, regardless of which shebang was specified in other scripts, since those scripts are not the entrypoint of execution, but flask is. I didn't have to change the shebang in any of my scripts, just the flask executable.
How can you run Flask app which uses a specific version of python?
On my env "python" = python2.7 and "python3" = python3.6. I have installed Flask using pip3. I start the app with FLASK_APP=app.py flask run. It uses python2.7 to execute, I would like it to use python3.6.
Also tried adding #!flask/bin/python3 to app.py, but it did not have an effect.