ci,build: Add flake8, black, isort (and wire into CI)

[tony]

Requires #41 (pyproject.toml)

Changes

• Add [lint] (pip install -e '.[lint]') for flake8, isort, black

  This assures that normal packaging isn't bloated, but the dependencies can still be installed by developers

• Add 3 lint tools
  • flake8: python linting rules
  • black: formatting
  • isort: import order

• Lint configurations

  The configurations are stock, except isort and flake8 both have black profiles (docs)

• CI
  • tests: Add lint tools to test workflow
  • tests and pypi upload: actions/setup-python
  • v3 -> v3
  • Add pip cache

[twmr]

Thx a lot for that! I wanted to add a black/isort CI setup long ago!

[tony]

@thisch This is a nice one!