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

Use relative includes in Include/cpython/pyatomic.h for
pyatomic_gcc.h, pyatomic_std.h and pyatomic_msc.h.

Do a similar change in Include/cpython/pythread.h for
pthread_stubs.h include.

Author: zanieb

Include/cpython/pyatomic.h

@@ -574,15 +574,15 @@ static inline void _Py_atomic_fence_release(void);
 
 #if _Py_USE_GCC_BUILTIN_ATOMICS
 #  define Py_ATOMIC_GCC_H
-#  include "cpython/pyatomic_gcc.h"
+#  include "pyatomic_gcc.h"
 #  undef Py_ATOMIC_GCC_H
 #elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
 #  define Py_ATOMIC_STD_H
-#  include "cpython/pyatomic_std.h"
+#  include "pyatomic_std.h"
 #  undef Py_ATOMIC_STD_H
 #elif defined(_MSC_VER)
 #  define Py_ATOMIC_MSC_H
-#  include "cpython/pyatomic_msc.h"
+#  include "pyatomic_msc.h"
 #  undef Py_ATOMIC_MSC_H
 #else
 #  error "no available pyatomic implementation for this platform/compiler"

Include/cpython/pythread.h

@@ -22,7 +22,7 @@ PyAPI_DATA(const long long) PY_TIMEOUT_MAX;
     */
 #   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"