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

首先，在 Python 3.14 中重現針對使用 _pack_ = True 但沒有 _layout_ 的 struct 定義所產生的 DeprecationWarning，重點關注報告中描述的無位元欄位情況。檢視相關的 ctypes 行為以及報告中連結的 NumPy issue；完成的要求是確定一個一致認可的 layout 預設值，或明確決定該警告仍然是必要的。