Modernize codebase

[pradyunsg]

Basically, bringing this up to speed with most of my other OSS projects.

• Adopt nox for development workflows.
  • nox -s lint
  • nox -s test-M.N
  • nox -s docs
• Adopt pre-commit for linting.
  • Blacken-ed the code.
  • isorted the code.
  • enforce pydocstyle
  • enforce newlines at the end of files.
• Switch to flit for packaging.
  • We don't use any setuptools specific stuff, and flit is simpler to use overall.
• Switch to src/ layout.
  • See https://github.com/pypa/packaging.python.org/issues/320 for context. (yes, I know it is blocked on me doing $thing)
• Cleans up docs/conf.py
  • Defer to sphinx's documentation instead of holding 100s of lines of comments.
  • Eventual plan is to host documentation on RTD.
• Move tests out of the shipped code.
  • Primarily opinion based
  • I think it is nice to separate tests from code, especially when both are big/spread across (eg: pip shipping tests actually would increase PyPI costs). This isn't a big-impact situation, but I'd appreciate having consistency between this and (basically) all the other OSS packages I work on.
• Drop fabfile.py and other fabric-related files.
• Drop requirements*.txt files
  • See https://caremad.io/posts/2013/07/setup-vs-requirement/ for context.

No code changes here, beyond adding some not-so-great-but-better-than-nothing documentation strings to make pydocstyle happy.

[pradyunsg]

Q: Can we drop coveralls integration? Q: Can we drop requires.io integration?

[pradyunsg]

Let's do CI fixes in a follow up. I really don't want to wrangle with Travis CI right now.