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
• Calling dataclasses.dataclass imperatively (as opposed to as a decorator) produces errors astral-sh/ty#1705
• Accessing a class attribute via an instance produces Unknown type astral-sh/ty#1706
• Perplexing "Object of type typing.Any has no attribute" astral-sh/ty#1744
• Incomplete narrowing with terminal statement inside nested ifs astral-sh/ty#685
• improve typevar solving astral-sh/ty#623
• Infer lambda expression based on the surrounding context astral-sh/ty#181
• narrow from union to constrained typevar type astral-sh/ty#2782

[reaperhulk]

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