Rather than having two separate ways to run the tests, this will make
tox the canonical way to do so. The one advantage invoking pytest
directly had over tox was that you could pass arguments to it (e.g.,
`-ksome_filter`), so the `py37` environment is gaining `{posargs}` to
allow passing parameters to pytest.

This reduces `test-requirements.txt` down to just tox. Rather than
creating a new requirements file for development dependencies (currently
only pip-tools), this will add a new tox environment, `freeze`, that can
be used to run `pip-compile`.