Update README and remove Python 2 support

[cbrnr]

Let's see if I can take this package to the present. This is a small start.

[cbrnr]

OK OK. This won't be that easy. I cannot even install the package in editable mode locally. Tests need to be converted to pytest. Travis needs to be replaced with GitHub Actions.

[mbillingr]

I exclusively use poetry now to manage dependencies and local development of Python packages. It uses pyproject.toml instead of setup.py/setup.cfg, which I find much more intuitive to maintain.

What do you think of switching?

[cbrnr]

Hi @mbillingr, nice to hear from you again 😃! I'd definitely switch to pyproject.toml, but I'd rather not use poetry. I'd prefer to stick with the official packaging tool pip, which also supports pyproject.toml nowadays: https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/, https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html

[mbillingr]

[pip] also supports pyproject.toml nowadays

nice! :)

[cbrnr]

Version and Python badges will be automatically updated based on the latest release on PyPI. So we should make one once we're done cleaning up!