Thanks jposada202020 for the PR!
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-si7021`.
Read the [docs](http://circuitpython.readthedocs.io/projects/si7021/en/latest/) for info on how to use it.