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Fixes #24922

Brief description of what is fixed or changed

This PR resolves partial issue https://github.com/sympy/sympy/issues/24922 by improving the code quality from using ".func is" to "isinstance()" in tests. 

Other comments

### Changes Made:
- Replaced `.func is` with `isinstance()` in relevant places in the codebase.

Release Notes

* core
   * Replaced `.func is` with `isinstance()` in relevant places in the codebase.
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These are actually semantic changes. Is this really a problem that needs fixing?

It was a code quality suggestion, also it maintain the uniformity in code base for further linting ease.

There is a code quality check that prohibits writing .func is outside of the test suite:

You might be right that it is fine in this case. But making sweeping (semantic) changes across the tests code to please a code linter will always run the risk of introducing unintended changes to the test cases.

I personally would skip the entry in the release notes, since it's only affecting test files. (so the change is not user facing).

After carefully looking though all the changes I think I agree that this is ok. Fine to merge by me.

But making sweeping (semantic) changes across the tests code to please a code linter will always run the risk of introducing unintended changes to the test cases.

I agree. Note in this case though that this is an "in-house" linter i.e. someone specifically implemented this rule for SymPy (without checking the history my guess would be Aaron).

One downside of using isinstance is that it won't work with SymEngine. On the other hand a typechecker requires these things to use isinstance e.g.:

p = Pow(x, 2)
assert isinstance(p, Pow) and p.exp == 2

I personally would skip the entry in the release notes

Agreed. There should be no release note here so it should have NO ENTRY instead.