Move away from pytest-runner

From pytest-runner's README:

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 tests_requires option.
• Select a tool to bootstrap and then run tests such as tox.