Implement a hiding mechanism for sticky banners

Maybe once it becomes sticky, it could shrink and show a shorter message?

I think the close button is good-enough as a solution for this TBH, because it's a more respectful way to allow users to hide banners as-and-when they wish to do so.

Originally posted by @pradyunsg in https://github.com/python/peps/issues/2992#issuecomment-1464897620

The specific UX might differ from the initial discussion referenced above, based on whether we want to completely hide the banner or just move it to a non-sticky state (avoiding a re-flow of the content on "close").

首先阅读 python/peps#2992 中链接的讨论，尤其关注有关关闭按钮、缩小以及避免内容重新排版的要点。明确横幅应该完全隐藏，还是改为 non-sticky，然后在定位实现之前定义预期的用户交互和验收行为。