Select Python runtime based on hashbang or syntax

When opening a file containing `#!/usr/bin/python3`, or containing obvious Python 3 only syntax, the Python3 runtime should be selected.

My first encounter opening a .py file was a prompt asking if I wanted to install PyLS, which sounded great. Then it failed because it tried to do that with Python 2, despite plenty of evidence that the file editor knew it contained Python 3 only syntax. PyLS fails on Python 2 because its dependencies (pylint) no longer install on python 2.

I needed to change my global? preferences to set the default Python to python3 before the PyLS would install and work.