Fix bug in selecting hdu indices in sunpy.io.fits.read

[wtbarnes]

Fixes #4181 and cleans up a few tests for sunpy.io.fits

[Cadair]

Can haz changelog?

[wtbarnes]

I added separate changelogs for the docfix and the bugfix. Do they need to be numbered as well? i.e. 4183.bugfix.1.rst and 4183.docfix.2.rst?

[ayshih]

Nope

[sunpy-backport]

The backport to 1.0 failed:

Commits ["461c8e082c8c1ce83df17487724c6a7187802e39","10ebf99c5079c4b90744f31eda2bf655028a0359","184f0dac9831a12e3f51d4e20dcb2bd1acb3f1eb","accc41a875a6da8d20e3e527b3fb9119705b322a","a654761cda3b8f108f810ba2b23268bae03d1a4d","cb8923842b3a2749bdff5dc05951a9a19d9ae7b5","12a0a61fed01f0dce8f73ef709c2e8aa0cb3c4f7"] could not be cherry-picked on top of 1.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 1.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 461c8e082c8c1ce83df17487724c6a7187802e39 10ebf99c5079c4b90744f31eda2bf655028a0359 184f0dac9831a12e3f51d4e20dcb2bd1acb3f1eb accc41a875a6da8d20e3e527b3fb9119705b322a a654761cda3b8f108f810ba2b23268bae03d1a4d cb8923842b3a2749bdff5dc05951a9a19d9ae7b5 12a0a61fed01f0dce8f73ef709c2e8aa0cb3c4f7
# Create a new branch with these backported commits.
git checkout -b backport-4183-to-1.0
# Push it to GitHub.
git push --set-upstream origin backport-4183-to-1.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 1.0 and the compare/head branch is backport-4183-to-1.0.