[3.10] bpo-44654: Do not export the union type related symbols (pythonGH-27223).

(cherry picked from commit 8f50f44)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Include/internal/pycore_unionobject.h

@@ -10,11 +10,11 @@ extern "C" {
 
 extern PyTypeObject _PyUnion_Type;
 #define _PyUnion_Check(op) Py_IS_TYPE(op, &_PyUnion_Type)
-PyAPI_FUNC(PyObject *) _Py_union_type_or(PyObject *, PyObject *);
+extern PyObject *_Py_union_type_or(PyObject *, PyObject *);
 
 #define _PyGenericAlias_Check(op) PyObject_TypeCheck(op, &Py_GenericAliasType)
-PyObject *_Py_subs_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
-PyObject *_Py_make_parameters(PyObject *);
+extern PyObject *_Py_subs_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
+extern PyObject *_Py_make_parameters(PyObject *);
 
 #ifdef __cplusplus
 }