Remove deprecated call to astropy helpers.

[wtgee]

Removes the function call and defaults to pytest just showing the warning at end of test run. Thanks @pllim for pointing out.

Edit: Astropy issue here https://github.com/astropy/astropy/issues/9718 and PR https://github.com/astropy/astropy/pull/14670

Closes #553

P.S. I feel like there is some kind of meta irony in us not noticing this was deprecated...