MAINT/CI: drop pint in favor of sympy, various CI tweaks

[ZLLentz]

Description

Try to replace pint with sympy to see if we have more stable test suites. Various other CI adjustments related to numpy 2.0

Motivation and Context

We've historically had some compatibility issues with pint every time a major package has an API change. For example: right now, numpy has updated to 2.0 and an optional dependency of pint is not importable with numpy 2 and therefore it busts the entire test suite.

Let's stop using it because we don't need all the features and this pint.compat module has been a constant thorn in our side.

Candidate 1: raw sympy (here) Candidate 2: possibly unyt? (separate PR?)

How Has This Been Tested?

The tests still pass for our existing unit conversions

Where Has This Been Documented?

Here only

Pre-merge checklist

• [x] Code works interactively
• [x] Code contains descriptive docstrings, including context and API
• [x] New/changed functions and methods are covered in the test suite where possible
• [x] Test suite passes locally
• [x] Test suite passes on GitHub Actions
• [x] Ran docs/pre-release-notes.sh and created a pre-release documentation page
• [x] Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I'm going to get this in so the CI will start being useful in the other PRs, even though I think there are a couple small things to improve here.