gh-144490: Fix test_cppext on Windows (#144628)

Don't include pycore_backoff.h and pycore_cell.h on Windows, since
they emit C++ compiler warnings.

Author: vstinner

Lib/test/test_cppext/extension.cpp

@@ -16,10 +16,9 @@
    // gh-135906: Check for compiler warnings in the internal C API
 #  include "internal/pycore_frame.h"
    // mimalloc emits many compiler warnings when Python is built in debug
-   // mode (when MI_DEBUG is not zero)
-   // mimalloc emits compiler warnings when Python is built on Windows
-   // in free-threaded mode.
-#  if !defined(Py_DEBUG) && !(defined(MS_WINDOWS) && defined(Py_GIL_DISABLED))
+   // mode (when MI_DEBUG is not zero).
+   // mimalloc emits compiler warnings when Python is built on Windows.
+#  if !defined(Py_DEBUG) && !defined(MS_WINDOWS)
 #    include "internal/pycore_backoff.h"
 #    include "internal/pycore_cell.h"
 #  endif