MAINT: parse ndarray.flags keys as unicode directly (gh-15317) - #32305

PR summary

This is one of the code that's trying to resolve numpy#15317 which is to take out the code as below

if (PyUnicode_Check(obj)) {
    PyObject *obj_bytes = PyUnicode_AsASCIIString(obj);   /* encode first */
    ...
}
PyBytes_AsStringAndSize(obj, &str, &length);              /* then work with bytes */

which is all over this repo. This is problematic as PyUnicode_AsASCIIString(obj); could throw UnicodeEncodeError, which does not describe the issue accurately. In this PR, I'm replacing one for the function getitem to have simply key errors instead of encoding errors. For the function setitem, it would have the exact same behavior, but we're removing the PyUnicode_AsASCIIString function in the code and simplifying the code.

AI Disclosure

Code, tests, and release notes are AI-generated (Claude model Opus). PR summaries and comments are mine. I've looked into the code extensively, so I should know what's going on fully.