Switch from mypy to ty

[reaperhulk]

Tracking known blockers for us to migrate.

• pytest.skip (and variants) are NoReturn but not properly detected astral-sh/ty#1112
• incorrect type narrowing when using pytest.raises astral-sh/ty#783

[reaperhulk]

#13656 drops us to 32 issues (most/all of which are ty limitations at the moment).