Replace uncompyle6 test with `inspect` and `ast.parse` or just drop it (Python 3.8 is end-of-lifed)

Although we can keep checking uncompyle6 releases, I think the state of the art is fixed at

Although we do not decompile bytecode greater than 3.8, code supports running from up to 3.12.

Our test_compute_features.py test uses uncompyle6 to get an AST of all the functions in the compute submodule and verify that they restrict themselves to a small set of features, so that there are fewer limitations on new backends. The new SymPy backend is also helping to keep the compute functions general, since the SymPy backend makes fewer assumptions than anything else (vector components aren't even numerical!), but it would be nice to keep directly checking.

Python 3.8 support ended over a month ago. But, rather than dropping the AST checks, what if we use Python's inspect module to find the source code lines and re-parse them with ast.parse?

Aha! It's because many of them are auto-generated. For instance, this for loop generates 48 functions via composition. Python's inspect can't find the source code because there never was any source code for these functions—they're made on-the-fly.

So if we want to preserve the test_compute_features.py tests (past the point when Python 3.8 can no longer be accessed in GitHub Actions), we'd have to generate source code text for these functions. One way to do that would be to use uncompyle6 to get AST and then use ast.unparse (or similar functionality from uncompyle6) to make source code text that we'll check into git instead of the generating functions. Or modify the generating functions to produce source code text directly. But, either way, we'd switch to managing 48 (not a big number) functions, rather than the for loop that generates them. As a side-bonus, error messages would report line numbers that exist in source code that you can look up.

Alternatively, we'll just remove test_compute_features.py when it becomes impossible to install Python 3.8 (in an easy way, without deadsnakes...)

Start with tests/test_compute_features.py and the generated functions in src/vector/_compute/lorentz/add.py. Review how uncompyle6 currently supports the AST checks and evaluate the proposed inspect/ast.parse or source-generation alternatives. Done means either preserving equivalent checks without Python 3.8 or removing the test with the project’s decision documented.