This should be uncontroversial enough, so merging without a review.

For future reference: tox sees only the root level conftest.py, while we need astrowidgets/conftest.py for the Astropy test runner, so that's why there are two copies. If we decide we don't need to run astrowidgets.tests(), we can remove the test runner from _astropy_init.py and also remove astrowidgets/conftest.py.