Improve CI with GitHub Actions and Wheels

[grantjenks]

trafficstars

Goals:

• [ ] Replace Travis CI with GitHub Actions
• [ ] Replace AppVeyor with GitHub Actions
• [ ] Drop Python 2 support
• [ ] Drop Python 3 < 3.6 support
• [ ] Publish Cythonized wheels
• [ ] Publish benchmarks showing Cython improvement
• [ ] Add mypy type checking to build process
• [ ] Merge rstcheck linting into tox
• [ ] Merge doc8 linting into tox
• [ ] Merge isort linting into tox
• [ ] Merge flake8 linting into tox
• [ ] Merge black into tox
• [ ] Drop SortedDict.iloc property

[grantjenks]

Reference: https://stackoverflow.com/questions/54117786/add-numpy-get-include-argument-to-setuptools-without-preinstalled-numpy

[grantjenks]

Reference: https://packaging.python.org/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/

[grantjenks]

We've got GitHub Actions now. I'll update tox soon.