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").

まず、python/peps#2992 のリンク先の議論を読み、特に閉じるボタン、縮小、コンテンツのリフローを避けることについての点を確認してください。バナーを完全に非表示にするのか、non-sticky にするのかを明確にし、その後、実装箇所を探す前に、想定されるユーザー操作と受け入れ時の動作を定義してください。