Drop pytest-runner use

[brews]

From https://pypi.org/project/pytest-runner/:

pytest-runner depends on deprecated features of setuptools and relies on features that break security mechanisms in pip. For example ‘setup_requires’ and ‘tests_require’ bypass pip --require-hashes. See also pypa/setuptools#1684.

It is recommended that you:

• Remove ‘pytest-runner’ from your ‘setup_requires’, preferably removing the setup_requires option.
• Remove ‘pytest’ and any other testing requirements from ‘tests_require’, preferably removing the setup_requires option.
• Select a tool to bootstrap and then run tests such as tox

I don't think dropping this would radically change the package, or how tests are run. It's just a matter of carefully removing it and everything created for it.