Update to Astropy #1588

This rebases astropy/astropy#1588 on the latest master, and it updates the PR to use setup/teardown_module fixtures.  This addresses the issue that was raised in the original PR of breaking tests in other modules (and perhaps in a simpler way than using a decorator as previously suggested).

Note: The tests for modeling won't pass with this change, as it does catch some areas where Numpy warnings are being raised.  Those should be addressed in a subsequent update.