MNT: Infrastructure and other updates

[pllim]

I stared into the code and I couldn't unsee things, so here it is.

• Fix #37 (you already have coverage so just adding a badge)
• Fix #133 (while we're at this, you don't really need photutils or matplotlib at runtime either)
• Fix #186
• Fix #189
• Fix #191 (was mostly there)
• Fix #198 (you want modernization, you are getting modernization)
• Apply astropy/astropy#15638
• Other fixes, especially tests that were not set up properly.
• Also looks like you have remote data tests but never used --remote-data flag in any of your CI job.
• And you also download a remote file blindly for tests not marked as remote data. I turned that file into a proper package test data that is local so you would stop spamming Box.

~Unable to handle this one until I get access:~

• 203

After merge, can add new job names to branch protection.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (aa4ceab) 78.83% compared to head (bbf377d) 81.32%.

Files	Patch %	Lines
specreduce/fluxcal.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #202 +/- ## ========================================== + Coverage 78.83% 81.32% +2.49% ========================================== Files 10 10 Lines 992 996 +4 ========================================== + Hits 782 810 +28 + Misses 210 186 -24 ```

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[pllim]

Close/reopen to see if RTD pops up.

[pllim]

Sorry about the conflict with #182 . I don't watch this repo so I was not aware of it. I replied in https://github.com/astropy/specreduce/pull/202#discussion_r1418278247 (I think it is a little too soon but I am also not a specreduce maintainer).

Whether you want to merge this first, or cherry-pick your changes to this PR and take this over, I will leave to you.

[tepickering]

no worries on the conflicts. i would say let's merge this ASAP and i'll do what i need to do to clean up #182. it's the code changes that i've made in #182 that require 3.10+, iirc. if we want to stick with bumping the requirement there vs here, i'm fine with that. interested in @kecnry's take.

[kecnry]

I don't have any strong opinions on the order of operations - good to merge on my end! Thanks for all the improvements @pllim !

[pllim]

Then, feel free to merge! I am tempted to merge myself but I shouldn't. Thanks!