Remove last need for setup.py

There only appears to be 1 vestigial need for ``setup.py``. Is there any way we can remove that and therefore the entire file?

https://github.com/astropy/astropy/blob/52f737bf955a1089acd238448a4925456f560ea1/setup.py#L72-L74

Start by examining setup.py, especially lines 72-74, to identify the remaining vestigial use. Confirm that this use can be removed without leaving any required setup behavior, then verify that setup.py itself can be deleted.

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