Avoid DeprecationWarning for `_pack_` without `_layout_` when there are not bit-fields?

Bug report

Bug description:

New Python versions give a DeprecationWarning for struct definitions that use _pack_ = True but do not specify the _layout_.

In a sense a new feature, but as I believe this is only relevant for bit-fields: wouldn't it make sense to allow leaving _layout_ unspecified when the struct contains no bitfields?
(I suppose that means giving it some default, such as the system native layout.)

The context is just seeing this in NumPy where bit-fields are not used so I was wondering: https://github.com/numpy/numpy/issues/28926.
Adding the _layout_ isn't a problem of course, but if the no-bitfield case is common (not sure) it may be nice to not be forced to "decide" on an irrelevant _layout_ (or act up on the DeprecationWarning)?

Another question that comes to mind is that _layout_ = "native" or so seems potentially useful.

CPython versions tested on:

3.14

Operating systems tested on:

Other

Comienza reproduciendo la DeprecationWarning en Python 3.14 para una definición de struct que use _pack_ = True sin _layout_, centrándote en el caso sin campos de bits descrito en el informe. Revisa el comportamiento relacionado de ctypes y el issue de NumPy enlazado en el informe; la tarea requiere que se acuerde un valor predeterminado para el layout o que se tome una decisión clara de que la advertencia sigue siendo necesaria.