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

まず、_layout_ なしで _pack_ = True を使用する struct 定義について、Python 3.14 で DeprecationWarning を再現します。レポートで説明されているビットフィールドなしのケースに焦点を当ててください。関連する ctypes の動作と、レポートでリンクされている NumPy issue を確認します。完了には、layout のデフォルト値について合意するか、警告が引き続き必要であるという明確な判断をすることが必要です。