Remove deprecated pytest-runner and setup.py test

The setup.py test command and pytest-runner have been deprecated for
several years and are no longer recommended. The pytest documentation
no longer recommends pytest-runner and their own documentation
recommends against it.

- https://github.com/pypa/setuptools/issues/1684
- https://github.com/pytest-dev/pytest/issues/5534
- https://github.com/pytest-dev/pytest-runner/issues/50

Changes made:

- Removed pytest-runner: Eliminated the use of pytest-runner from
  `setup.py`.
- Moved test dependencies to the `extra-dependencies`, which can be used
  by tox as well as for manual installation.
- Updated the documentation to recommend using tox as the default test
  runner. Alternatively, users can manually run tests with the `py.test`
  command.

These changes will also make an eventual migration to `pyproject.toml`
easier, as the `extras_require` can be copied to the
`project.optional-dependencies` section.