Feature/pyproject pypi

[AndydeCleyre]

I thought I'd have a go at updating this and preparing it for PyPI. Please have a look at these changes; I hope they're useful.

You should be able to build a package locally:

$ pip install '.[dev]'
$ flit build

Or build and publish to PyPI at once:

$ pip install '.[dev]'
$ flit publish

I dropped py2 support along the way...