`_operator._compare_digest` is incorrectly typed

[jonathandung]

When working on #15611, I realized that the internal _compare_digest function, which accepted two strings or anything that implements the buffer protocol (so-called 'bytes-like objects'), was annotated to only take two AnyStrs, the upper bound of which is str and bytes (though type checkers understand that bytearray is accepted also). Therefore, the following snippet, which prints True at runtime without fault, will fail type checking:

import _operator
class test:
    def __buffer__(self, flags, /): return memoryview(b'')
    def __release_buffer__(self, buffer, /): buffer.release()
print(_operator._compare_digest(test(), test()))

The argument annotation should be changed. Another TypeVar, constrained to str and collections.abc.Buffer, is likely needed.

[jonathandung]

See the C source:

static PyObject *
_operator__compare_digest_impl(PyObject *module, PyObject *a, PyObject *b)
/*[clinic end generated code: output=11d452bdd3a23cbc input=9ac7e2c4e30bc356]*/
{
    int rc;

    /* ASCII unicode string */
    if(PyUnicode_Check(a) && PyUnicode_Check(b)) {
        if (!PyUnicode_IS_ASCII(a) || !PyUnicode_IS_ASCII(b)) {
            PyErr_SetString(PyExc_TypeError,
                            "comparing strings with non-ASCII characters is "
                            "not supported");
            return NULL;
        }

        rc = _tscmp(PyUnicode_DATA(a),
                    PyUnicode_DATA(b),
                    PyUnicode_GET_LENGTH(a),
                    PyUnicode_GET_LENGTH(b));
    }
    /* fallback to buffer interface for bytes, bytearray and other */
    else {
        Py_buffer view_a;
        Py_buffer view_b;

        if (PyObject_CheckBuffer(a) == 0 && PyObject_CheckBuffer(b) == 0) {
            PyErr_Format(PyExc_TypeError,
                         "unsupported operand types(s) or combination of types: "
                         "'%.100s' and '%.100s'",
                         Py_TYPE(a)->tp_name, Py_TYPE(b)->tp_name);
            return NULL;
        }

        if (PyObject_GetBuffer(a, &view_a, PyBUF_SIMPLE) == -1) {
            return NULL;
        }
        if (view_a.ndim > 1) {
            PyErr_SetString(PyExc_BufferError,
                            "Buffer must be single dimension");
            PyBuffer_Release(&view_a);
            return NULL;
        }
...

Py_buffers are evidently created for non-strings.