Blacken code

[clbarnes]

https://github.com/ambv/black

To do:

• [ ] Set the stage by blackening everything in one fell swoop
• [ ] Add black to dev requirements
• [ ] Add black --check . to the tox config for python 3.6+
• [ ] Add pre-commit hook for running black in appropriate environments and warning users developing in an inappropriate environment

While we're at it, make lint can go in the pre-commit hooks too, if the CI's going to fail on it.

[clbarnes]

Shell one-liner to see if python is modern enough to run black

python -c "import platform, sys; sys.exit(tuple(int(v) for v in platform.python_version().split('.')) < (3,6))"

N.B. probably better to just have pip install black where appropriate and then have make lint do black && black --check .