precommit git hooks, rearrange package structure

[sumpfork]

This

• moves the package from the top level into a src/ subdir
• moves the tests out of the package to the top level
• adds pre-commit hooks to format the code via black, check it via flake8, and do some other formatting
• adds travis instructions to run the precommit hooks there, too
• properly separates development requirements into requirements.txt (produced via pip-compile from requirements.in)
• fixes all the formatting etc. according to the precommit hooks
• I haven't yet enabled/run the json formatting hook, but I think it'd be a good idea to stay consistent with all our json