Astropy: Deprecate enable_deprecations_as_exceptions

[pllim]

Description

As proposed by @mhvk in https://github.com/astropy/astropy/issues/9690#issuecomment-559154730 . The reasoning is that we can use pytest's filterwarnings in setup.cfg instead.

Additional context

This will affect all the other packages that use this feature as well, especially if they were created from our package template.

Once deprecation is in, a follow up issue needs to be opened for its complete removal after a deprecation period.

[bsipocz]

😱 - it's used downstream a lot, so it needs to be a long deprecation and/or batchPR-ed.

[astrofrog]

We could stop using it in the core package immediately though, long before we deprecate/remove it.

[pllim]

First stage: #10503