Thanks to jepler, we can now read signed bitfields without doing the math ourselves! Huzzah!
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-register`.
Read the [docs](http://circuitpython.readthedocs.io/projects/register/en/latest/) for info on how to use it.