Modernize packaging and CI (pyproject.toml, Tox, GHA)

[bittner]

Adds:

1. modern packaging setup with pyproject.toml file (replacing setup.py and setup.cfg)
2. Tox configuration for running checks, tests and packaging, anywhere
3. an initial test setup and a first, minimal test
4. a GitHub Actions CI setup in the .github folder (replacing .travis.yml)
5. instructions on running checks and tests (README)

Updates the build badge at the top of the project README.

Closes #40

[bittner]

The tests in this PR work for Python 3.10 through 3.12; for Python 3.9 and below the importlib module in test_staticfiles() is a bit unhappy.

What still needs to be added for the tests to be actually useful are tests with Django templates.