Allow installing just some of the dependencies. Some filters won't work,
but that's expected. Dcoument the change and keep all dependencies as
required for now to not cause unexpected breakage.

Dependabot in GitHub shows a warning, but these are just used for
testing and releasing (not affecting users in any way). Still, update
and close that warning.

Useful information. I will admit I'm bragging a bit with the download
counter. I didn't believe it would have so many downloads (I'm really
excited).

Now it runs the CLI as well. Duplicates other jobs but should be easier
to debug some cases.

Had to workaround the jinja package missing in some cases (like
pre-commit) so I can import the module to get the docstring.

- Fix missing output of bats in CI.
- Output doctests to junit only in CI.

- Use templates.
- junit output for doctest and bats.

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.