BUG: aperture sum counted npix twice

[pllim]

Description

This pull request is to fix a bug where aperture sum counted npix twice when converting from MJy/sr to MJy. Thanks to @orifox for finding this bug and @larrybradley for the fix suggestion!

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set?
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov]

Codecov Report

Merging #1332 (2e58202) into main (12d97b8) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1332      +/-   ##
==========================================
+ Coverage   84.60%   84.77%   +0.16%     
==========================================
  Files          91       91              
  Lines        7836     7835       -1     
==========================================
+ Hits         6630     6642      +12     
+ Misses       1206     1193      -13     

Impacted Files	Coverage Δ
...imviz/plugins/aper_phot_simple/aper_phot_simple.py	91.52% <100.00%> (-0.03%)	⬇️
...s/default/plugins/model_fitting/fitting_backend.py	93.02% <0.00%> (+15.11%)	⬆️

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[orifox]

All the new numbers match mine. I approve. :)

[pllim]

Thanks, all!