[BUG]: pybind11 2.10.0 - error_fetch_and_normalize changes conflicts with PyPy behavior for PyErr_NormalizeException

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Problem description

PyPy 3.8 triggers the newly introduced error message

https://github.com/pybind/pybind11/blob/59f03ee389c283cde65bd800c8f32ea690daf3fd/include/pybind11/pytypes.h#L459-L469

MISMATCH of original and normalized active exception types: ORIGINAL OSError REPLACED BY FileNotFoundError: [Errno 2] No such file or directory: 'this_file_does_not_exist'

In PyPy, PyErr_NormalizeException causes the "raw" OSError to be normalized into FileNotFoundError. Which seems fair enough.

It's unclear to me whether PyPy is doing something "naughty" here that violates the expectations set by CPython, or if this new exception handling code is just imposing an expectation that doesn't hold for PyPy. As such I don't know if simply disabling that test is appropriate for PyPy.

CPython does not make the same substitution. There is no error (and the code behaves correctly) with pybind11 2.9.

Reproducible example code

    m.def("test_pypy_oserror_normalization", []() {
        // We want to do "with suppress(FileNotFoundError): open(f)" in C++
        try {
            py::module_::import("io").attr("open")("this_file_does_not_exist", "r");
        } catch (const py::error_already_set &e) {
            // FileNotFoundError
        }
    });

Start with include/pybind11/pytypes.h at the linked error handling around PyErr_NormalizeException, then reproduce the provided C++ example under PyPy 3.8 and CPython. Determine whether PyPy's OSError-to-FileNotFoundError normalization is expected, and document or implement compatibility behavior with regression coverage; done means the mismatch is resolved without breaking the existing CPython behavior.