Use tabs for generics docs

[cdce8p]

Followup to

• Document new type parameter syntax (PEP 695) #17810
• Add sphinx_inline_tabs to docs #18262

Reformat docs to use sphinx_inline_docs for generics documentation to differentiate between legacy and Python 3.12+ syntax. Temporarily use custom requirement to be able to disable the tab sync. It might be desirable to compare both versions side by side. With the tab sync (the default), each switch would move the text around since the tab lengths are different. With the sync disabled, at least the top doesn't jump around.

https://github.com/cdce8p/sphinx-inline-tabs/releases/tag/2024.12.07.b1
cdce8p-forks/sphinx-inline-tabs@main...2024.12.07.b1

[cdce8p]

Is this something we'd want to add? @JukkaL you mentioned in #17810 (comment) that you wanted to see how it looks first. What do you think?

[JukkaL]

It looks good to me. I think the initial code examples in the generic section can remain as is, so that the reader sees both syntax variants without having to switch tabs, but later on we can use tabs.

However, I don't love the tab title "Legacy". What about "Old syntax" instead?