From the digging I've been doing, it seems like the way to get pytest to add your local code to sys.path is to place a conftets.py and pytests.ini file in the base directory of your project. Since I haven't been able to find a mention of this in the documentation I'm hoping a dev can shed some light on whether this is a supported feature or not, and if there's a more canonical way of doing it.

Relevant SO posts:
http://stackoverflow.com/questions/34466027/in-py-test-what-is-the-use-of-conftest-py-files
http://stackoverflow.com/questions/39499937/is-there-a-standard-way-to-tell-py-test-to-run-against-specific-code