`Container.__contains__` should be generic

```py
@runtime_checkable
class Container(Protocol[_T_co]):
    # This is generic more on vibes than anything else
    @abstractmethod
    def __contains__(self, x: object, /) -> bool: ...
```
>>>
```py
@runtime_checkable
class Container(Protocol[_T_co]):
    @abstractmethod
    # must implement safe variance
    def __contains__(self, x: _T_co, /) -> bool: ...  # type: ignore[unsafe-variance]
```
