v0.3.1 broken by new version of setuptools

[chillenb]

Version 72 of setuptools removes setuptools.command.test, which is used in setup.py in v0.3.1. Therefore, users with the newest version of setuptools cannot install cppe from PyPI. See the pyscf CI log for an example.

[maxscheurer]

Thanks for the heads-up! Will be fixed via #44 hopefully soon 👍🏻