[BUG]: KeysView/ItemsView does not support set operations

[InvincibleRMC]

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What version (or hash if on master) of pybind11 are you using?

master

Problem description

When calling .items()/.keys() on the result of bind_map<>() return a custom KeysView/ItemsView which don't implement set operations. Ref. Looking at the history there seems to be a lot of issues/pull requests around them. I assume there is some history about why they don't use collections.abc.KeysView/collections.abc.ItemsView type at runtime and have a custom implementation.

Reproducible example code

No set operations exist. See #5877 for an example of 1.

Is this a regression? Put the last known working version here if it is.

Not a regression