MNT: NumPy 2.0 compatibility, other fixes

[pllim]

Description

This pull request is to fix #362 and any resulting modernization that has to happen.

[github-actions]

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• Do the proposed changes actually accomplish desired goals?
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[github-actions]

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[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
synphot/observation.py	100.00%
synphot/spectrum.py	100.00%

📢 Thoughts on this report? Let us know!.

[pllim]

Phew... Not too bad.

[pllim]

I cannot fix stsynphot easily without merging this, so just gonna merge...