gh-129296: Fix `pythread.h` include paths (#129320)
Use relative includes in Include/cpython/pythread.h for
pthread_stubs.h.
(cherry picked from commit
3a974e39d54902699f360bc4db2fd351a6baf3ef)
Co-authored-by: Zanie Blue <contact@zanie.dev>
*/
# 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"