Issue #25150: Hide the private _Py_atomic_xxx symbols from the public
Python.h header to fix a compilation error with OpenMP. PyThreadState_GET()
becomes an alias to PyThreadState_Get() to avoid ABI incompatibilies.

It is important that the _PyThreadState_Current variable is always accessed
with the same implementation of pyatomic.h. Use the PyThreadState_Get()
function so extension modules will all reuse the same implementation.
diff --git a/Include/pyatomic.h b/Include/pyatomic.h
index 99816a5..892a217 100644
--- a/Include/pyatomic.h
+++ b/Include/pyatomic.h
@@ -1,8 +1,6 @@
-/* Issue #23644: <stdatomic.h> is incompatible with C++, see:
-   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60932 */
-#if !defined(Py_LIMITED_API) && !defined(__cplusplus)
 #ifndef Py_ATOMIC_H
 #define Py_ATOMIC_H
+#ifdef Py_BUILD_CORE
 
 #include "dynamic_annotations.h"
 
@@ -248,5 +246,5 @@
 #define _Py_atomic_load_relaxed(ATOMIC_VAL) \
     _Py_atomic_load_explicit(ATOMIC_VAL, _Py_memory_order_relaxed)
 
+#endif  /* Py_BUILD_CORE */
 #endif  /* Py_ATOMIC_H */
-#endif  /* Py_LIMITED_API */