Write union types as X | Y where possible

[AndrewFerr]

aka PEP 604, added in Python 3.10

This PR has been broken down into multiple commits, to illustrate the steps used to prepare it.

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[AndrewFerr]

The trial-olddeps failure is caused by using Pydantic v1, which won't support Python 3.10: pydantic/pydantic#2607 (comment)

Thankfully, there's already a PR for enforcing Pydantic v2: #19071

[AndrewFerr]

The Pydantic update indeed fixed trial-olddeps.

I believe the current Complement test failure (TestPartialStateJoin/joined_members_blocks_during_partial_state_join) is a false-positive, as I recall it sometimes failing in other unrelated PRs.