API changes towards a future removal of astropy test runner:

* Removed ``astropy.tests.command`` module that was deprecated in v6.0.
* Officially deprecated ``astropy.test``, ``astropy.tests.runner.TestRunnerBase``, and ``astropy.tests.runner.TestRunner`` (previously pending deprecation). This will also affect downstream ``packagename.test`` generated using ``TestRunner``. The deprecated functionality will be removed in a future release after a deprecation period as per Astropy deprecation policy.
