feat: create a virtualenv to manage runtime and dev dependencies

[brydoncheyney]

This PR will add support for the use a of virtualenv to manage project dependencies across development environments.

• install pyenv
• install pipenv
• pipenv install --dev
• pipenv run python ./PaintMixingGUI.py

I've also bundled a set of development/test packages to support general project hygiene, and a Makefile with targets to use

; make
help           Display this help
targets        Display this help
list           Display this help
pipenv         installs all python packages
lint           performs linting for python tooling
deadcode       checks for unused code in python tooling
typing         runs static type checking on python tooling
format-check   checks formatting on python tooling
test           runs pytest
coverage       generates markdown report of pytest coverage
all            applies all python checks

[miciwan]

Thanks for that! Useful stuff, as I have totally zero more production-oriented python experience!