Move away from setup.py to pyproject.toml

[john-science]

The Python org has said that pyproject.toml will be the new way to package ARMI code, going forward.

So we should do that, as we did in ARMI.

[john-science]

@AidanMcDonald This could be me or you. It doesn't have to be you, if you don't want.

[AidanMcDonald]

I have attempted to migrate to pyproject.toml, but I am new to this and I'm not sure I tested it correctly. I'd appreciate if you gave it a quick check before we close the issue

[ntouran]

FYI here's a tutorial on this topic: https://packaging.python.org/en/latest/tutorials/packaging-projects/