update container annotations

[jku]

Once #2721 is merged, we could require python 3.8 and use the nicer annotations and features. Known improvements:

• annotations for standard collections: dict instead of typing.Dict
• dict unions (I believe we have at least one place where this would be useful)

Vague process:

• bump requires-python in pyproject.toml
• run ruff check --fix --unsafe-fixes tuf tests examples
• review that changes made by ruff are sensible
• check if RootVerificationResult signed() and unsigned() work with a dict union -- this step could be left to another PR
• make sure tox -e lint and tox -e test pass

[jku]

• str | None instead of Optional[str]

Actually this might be in 3.10 only -- if that is true then we don't want to change it yet