Update `pyproject.toml` to include the ability optional dependencies. Note that these are not currently used, but are intended to keep this libraries aligned with others in the Adafruit CircuitPython Bundle. Thanks tekktrik!
To use in CircuitPython, simply install the [Adafruit CircuitPython Bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-bme680`.
Read the [docs](http://circuitpython.readthedocs.io/projects/bme680/en/latest/) for info on how to use it.