bpo-35081: Move Include/pyatomic.c to Include/internal/ (GH-10239)

Add pyatomic.h to the VS project (it wasn't referenced).
diff --git a/Include/internal/ceval.h b/Include/internal/ceval.h
index cdabb95..4297b5a 100644
--- a/Include/internal/ceval.h
+++ b/Include/internal/ceval.h
@@ -4,7 +4,7 @@
 extern "C" {
 #endif
 
-#include "pyatomic.h"
+#include "internal/pyatomic.h"
 #include "pythread.h"
 
 struct _pending_calls {