Use Stash instead of monkeypatching pytest.Item

[The-Compiler]

Maybe

    item: pytest.Item,

and add a type ignore about us monkeypatching stuff on it? Long-term, we should be using Stash instead anyways.

Same also for the other utility methods taking item below.

Originally posted by @The-Compiler in https://github.com/pytest-dev/pytest-qt/pull/605#discussion_r2153787063