Compatibility with specutils 1.5 and astropy 5.0

[gully]

I just spot-checked muler provisioned with specutils 1.5 and astropy 5.0. Remarkably it appears to work right out of the box! It even works with gwcs 0.17.1, which was previously disallowed for our muler install because of issue #70 .

This ease of install is a good thing. It raises the issue of how to handle backwards compatibility. Should we demand that users upgrade to specutils 1.5, gwcs 0.17.1, and astropy 5? That may have some backwards breaking changes for folks. It also will probably break whatever conda environment they currently have.

On the other hand, how to we support some gwcs-specutils-astropy combinations and not others? Maybe the easiest thing to do is disallow gwcs 0.17.0, and all other astropy-specutils combos should just work? Not sure! I suppose we could add gwcs to our existing build matrix.

We may also want to write some guidance in the docs on how to make a fresh conda environment if you're upgrading to specutils.

By the way, the tests are only failing due to an experimental commit to the equivalent_width method.

[gully]

I think our setup.py file may be misguided at the moment. It currently has install_requires with:

        "numpy",
        "scipy",
        # "astropy>=4.1,<5.0",
        "specutils",
        "pandas",
        "importlib_resources",
        "matplotlib",

We probably want to update it to:

        "numpy",
        "scipy",
        "astropy>=4.1",
        "specutils",
        "gwcs >=0.17.1",
        "pandas",
        "importlib_resources",
        "matplotlib",

That's what the new conda-forge requirement looks like.