bpo-35134: Add Include/cpython/longobject.h (GH-29044)

Move Include/longobject.h non-limited API to a new
Include/cpython/longobject.h header file.

Move the following definitions to the internal C API:

* _PyLong_DigitValue
* _PyLong_FormatAdvancedWriter()
* _PyLong_FormatWriter()

Python/mystrtoul.c

diff --git a/Python/mystrtoul.c b/Python/mystrtoul.c
index 19fa57a..e6fe154 100644
--- a/Python/mystrtoul.c
+++ b/Python/mystrtoul.c
@@ -1,5 +1,5 @@
-
 #include "Python.h"
+#include "pycore_long.h"          // _PyLong_DigitValue
 
 #if defined(__sgi) && !defined(_SGI_MP_SOURCE)
 #define _SGI_MP_SOURCE