bpo-40422: Move _Py_closerange to fileutils.c (GH-22680)

This API is relatively lightweight and organizationally, given that it's
used by multiple modules, it makes sense to move it to fileutils.

Requires making sure that _posixsubprocess is compiled with the appropriate
Py_BUIILD_CORE_BUILTIN macro.

Include/internal/pycore_fileutils.h

@@ -48,6 +48,8 @@ PyAPI_FUNC(int) _Py_GetLocaleconvNumeric(
     PyObject **decimal_point,
     PyObject **thousands_sep);
 
+PyAPI_FUNC(void) _Py_closerange(int first, int last);
+
 #ifdef __cplusplus
 }
 #endif