pythongh-105858: Expose some union-related objects as internal APIs

We now use these in the AST parsing code after pythongh-105880. A few comparable types (e.g.,
NoneType) are already exposed as internal APIs.

Include/internal/pycore_unionobject.h

@@ -8,9 +8,11 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-extern PyTypeObject _PyUnion_Type;
+// For extensions created by test_peg_generator
+PyAPI_DATA(PyTypeObject) _PyUnion_Type;
+PyAPI_FUNC(PyObject *) _Py_union_type_or(PyObject *, PyObject *);
+
 #define _PyUnion_Check(op) Py_IS_TYPE((op), &_PyUnion_Type)
-extern PyObject *_Py_union_type_or(PyObject *, PyObject *);
 
 #define _PyGenericAlias_Check(op) PyObject_TypeCheck((op), &Py_GenericAliasType)
 extern PyObject *_Py_subs_parameters(PyObject *, PyObject *, PyObject *, PyObject *);