RUF012 should allow plain ClassVar

[bluetech]

Running ruff 0.0.282 using ruff --isolated --select=RUF012 on the following file:

from typing import ClassVar

class C:
    f: ClassVar = {0}

gives

x.py:4:19: RUF012 Mutable class attributes should be annotated with `typing.ClassVar`

I think Ruff should not emit RUF012 for this, since if ClassVar is used without the [...] part, the type checkers infer the type, i.e. the [...] part is optional, similar to Final.