GeezerVison

Add _testcapi bindings and tests for the object and reason accessors of the PyUnicodeError C API:

• PyUnicode{Encode,Decode,Translate}Error_GetObject
• PyUnicode{Encode,Decode,Translate}Error_GetReason
• PyUnicode{Encode,Decode,Translate}Error_SetReason

The matching start and end accessors gained tests in gh-123378, which fixed the clamping of their return values. The object and reason accessors were not part of that change. Six of the functions above were never called by the test suite at all, and for the remaining three, every in-tree caller passes a genuine exception object, so the TypeError raised for a non UnicodeError argument was unreachable in all nine cases.

Objects/exceptions.c is already checked off in the gh-94808 checklist, but that predates the check_unicode_error_type() wrappers added in gh-127691 and gh-127787, which introduced the type check branch these tests exercise.

This adds only _testcapi bindings and tests. There is no behaviour change to Objects/exceptions.c.