Use 'python3' to access Python

[sebrowne]

As per https://peps.python.org/pep-0394, it is best practice to not use python in shebangs. Use the explicit python3 entry point instead.

Closes #607

I did not test this manually, I was planning on seeing how it works within the automated tests.

This is technically a break in backwards compatibility, but may not be experienced given how long Python 2 has been out of support.