runtime-cast-value (TC006) shouldn't apply to stubs

[Avasam]

Found this whilst applying various Ruff groups on typeshed.

According to the updated typing spec for typing enums (see Mypy: Change to Enum Membership Semantics and https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members, this is the recommended way to type an enum in stubs where the type is known but the value isn't:

class Key(enum.Enum):
    alt = cast(KeyCode, ...)
    alt_l = cast(KeyCode, ...)
    alt_r = cast(KeyCode, ...)
    alt_gr = cast(KeyCode, ...)
    # ...

This will trigger runtime-cast-value (TC006)

Related: #15132

Ruff: 0.8.4
Command: ruff check --preview --select=TC006 --isolated

[Daverball]

My bad, some of the other new flake8-type-checking rules I added might also currently be active in stub files. None of them should really be active.