Remove setup.py from "publish" target

[pganssle]

This replaces setup.py upload (which is deprecated and has caused problems for this library before) with twine upload, which is the modern way to do it.

I have also gone ahead and set up PEP 517/518 for this project, and used pep517.build to build the source and binary distributions, thus removing the need to invoke setup.py at all.

This both adds pyproject.toml and switches from setup.py to twine, but if you just want the twine command part of it, the first commit doesn't include the pyproject.toml changes.

[coveralls]

Coverage decreased (-0.8%) to 92.969% when pulling 90b8e2a on pganssle:bad_publish into c2d1dd2 on spulec:master.

[pganssle]

I think the reduction in coverage may be an artifact. I have not touched any .py files, so presumably Makefile or pyproject.toml are being erroneously included in the calculation of coverage percentage, and the fact that they have gotten larger makes the "percentage covered" seem like it fell.

Also, do you want me to add twine and pep517 to requirements.txt? They are only necessary when making releases, which is why I wasn't sure if they should go in the requirements file.

[boxed]

@pganssle I tend to ignore that check anyway because it's not hugely relevant as you say :)