Switch to Twine uploads for PyPI

[tripleee]

This should get rid of the legacy warnings when uploading to PyPI, and offer some nice goodies such as support for displaying the README.md in the project's description on PyPI.

See #23 (comment) for a surrounding discussion.

The main discussion point here is whether switching from distutils to setuptools is acceptable. There are some other dependencies which this change will pull in, but nothing which a modern Python 3 project would not usually require. I say let's go for it.

Show of thumbs please?

[tripleee]

https://gist.github.com/gboeing/dcfaf5e13fad16fc500717a3a324ec17 has a succinct summary of the requirements and commands (with some caveats; see comments).

[tripleee]

In fact install_requires in setup.py is apparently already a setuptools thing?

[double-fault]

This surely seems to be a good option.