Migrate nose to pytest

[clairekope]

Nose is no longer being developed which is causing issues with the newer version of Pythons required for CircleCI to function (#222)

[clairekope]

For now, it seems like the nose3 package (a fork of the original nose) will serve as a drop-in replacement. It hasn't seen an update in a year, and only supports up to Python 3.10.