[3.12] gh-129296: Fix pythread.h include paths (#129320) (#130668)

gh-129296: Fix `pythread.h` include paths (#129320)

Use relative includes in Include/cpython/pythread.h for
pthread_stubs.h.

(cherry picked from commit 3a974e3)

Co-authored-by: Zanie Blue <[email protected]>

Author: vstinner

Include/cpython/pythread.h

@@ -21,7 +21,7 @@ PyAPI_FUNC(int) _PyThread_at_fork_reinit(PyThread_type_lock *lock);
     */
 #   define NATIVE_TSS_KEY_T     unsigned long
 #elif defined(HAVE_PTHREAD_STUBS)
-#   include "cpython/pthread_stubs.h"
+#   include "pthread_stubs.h"
 #   define NATIVE_TSS_KEY_T     pthread_key_t
 #else
 #   error "Require native threads. See https://bugs.python.org/issue31370"