cext: fix for PyLong refactoring in CPython 3.12

Compiling Mercurial with Python 3.12 a5 would fail with:

mercurial/cext/dirs.c: In function '_addpath':
mercurial/cext/dirs.c:19:44: error: 'PyLongObject' {aka 'struct _longobject'} has no member named 'ob_digit'
   19 | #define PYLONG_VALUE(o) ((PyLongObject *)o)->ob_digit[0]
      |                                            ^~
mercurial/cext/dirs.c:97:25: note: in expansion of macro 'PYLONG_VALUE'
   97 |                         PYLONG_VALUE(val) += 1;
      |                         ^~~~~~~~~~~~
mercurial/cext/dirs.c:19:44: error: 'PyLongObject' {aka 'struct _longobject'} has no member named 'ob_digit'
   19 | #define PYLONG_VALUE(o) ((PyLongObject *)o)->ob_digit[0]
      |                                            ^~
mercurial/cext/dirs.c:108:17: note: in expansion of macro 'PYLONG_VALUE'
  108 |                 PYLONG_VALUE(val) = 1;
      |                 ^~~~~~~~~~~~
mercurial/cext/dirs.c: In function '_delpath':
mercurial/cext/dirs.c:19:44: error: 'PyLongObject' {aka 'struct _longobject'} has no member named 'ob_digit'
   19 | #define PYLONG_VALUE(o) ((PyLongObject *)o)->ob_digit[0]
      |                                            ^~
mercurial/cext/dirs.c:145:23: note: in expansion of macro 'PYLONG_VALUE'
  145 |                 if (--PYLONG_VALUE(val) <= 0) {
      |                       ^~~~~~~~~~~~

This was caused by
https://github.com/python/cpython/commit/c1b1f51cd1632f0b77dacd43092fb44ed5e053a9 .

mercurial/cext/dirs.c

@@ -13,7 +13,11 b''
 
 #include "util.h"
 
+#if PY_VERSION_HEX >= 0x030C00A5
+#define PYLONG_VALUE(o) ((PyLongObject *)o)->long_value.ob_digit[0]
+#else
 #define PYLONG_VALUE(o) ((PyLongObject *)o)->ob_digit[0]
+#endif
 
 /*
  * This is a multiset of directory names, built from the files that