tox environments for building packages and uploading to pypi

[lengau]

It would be nice if I could run something like:

tox -m publish to build the sdist and any necessary wheels and then upload them to pypi using twine.

[lengau]

Some basic ideas for implementation:

1. A build environment that could later get various build factors if needed (e.g. different Python versions if we end up building version-specific wheels)
2. A twine environment that runs twine upload
3. Possibly have the twine environment run twine check before uploading, failing if the check fails?
4. Make sure the environment that does the upload depends on the build environment(s)
5. A label that runs all those environments, allowing the dependencies to figure out order