For now, until I find a secure way to sing them in GitLab.

Remove Python versions that aren't tested anymore, add those that are
tested.

Shouldn't have beem merged this way.

Also:

- Update the README.
- Test more environments (every supported CPythong and PyPy version and
CPython 2.7).

- Only install bumpversion on Python 3.6 or later, don't install when
using Python 2.7 (I'm only using it locally and it's not needed when
testing).
- Remove dbus-python and keyring. They were optional requirements for
twine, but dbus-python is deprecated and keyring is now required.
- Don't require a specific Python version, in CI I'm testing with all
supported versions of Python and PyPy and Python 2.7.

Don't have multiple actions in the same script.

Ignore a few warnings, change the order of imports. Nothing material.

Since Python3.4 is EOL and Python3.5 has `run` in the `subprocess`
module, there's no need for it outside of Python2.7.

- 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.

- Added docs to the usual (local) Tox run.

- Replaced test shell script with Bats tests, update Travis CI and README
  accordingly.
- Removed unused pre-commit hooks.
- Bumped patch version.

- Fixed single issue found (autoescaping Jinja templates).
- Added parallel pre-commit, bandit tests in TravisCI.
- Bumped patch version.

- Moved the setup.py metadata check back to the Tox test environment, left just the restructuredtext check in the Tox docs environment.