Major overhaul of the project.

- Use pipenv instead of tox.
- Expand use of pre-commit, many more hooks.
- Run just Bats and doctest for each Python version, pre-commit and
twine check just with Python 3.7.
- Fix doctests, issues found by different checks.
- Use bumpversion instead of relying on a VERSION file.
- Update documentation.
- State AGPL license everywhere (stated MIT in setup.py by accident).
- Format code with Black.
- Test on Python 3.7 in Travis CI.