Thanks, @ebolyen! Yep, I hadn't updated my package since everything switched to Python 3.6.
I just re-built the package with Python 3.6 and uploaded it to anaconda, so you should be able to update the package with conda directly and everything will work as expected.
For posterity's sake, what @ebolyen meant by "installing from source" is to clone the repo and then run python setup.py install directly:
git clone https://github.com/cduvallet/q2-perc-norm.git
cd q2-perc-norm
python setup.py install