gh-91320: Fix more old-style cast warnings in C++ (#93285)

Use _PyObject_CAST() in the public C API to fix C++ compiler
warnings: "use of old-style cast" (clang -Wold-style-cast).

Include/pyerrors.h

@@ -62,7 +62,7 @@ PyAPI_FUNC(void) PyException_SetContext(PyObject *, PyObject *);
 
 PyAPI_FUNC(const char *) PyExceptionClass_Name(PyObject *);
 
-#define PyExceptionInstance_Class(x) ((PyObject*)Py_TYPE(x))
+#define PyExceptionInstance_Class(x) _PyObject_CAST(Py_TYPE(x))
 
 #define _PyBaseExceptionGroup_Check(x)                   \
     PyObject_TypeCheck(x, (PyTypeObject *)PyExc_BaseExceptionGroup)