Should be able to use covariant type on parameter on classmethod

```py
from typing import *
out_T = TypeVar("out_T", covariant = True)

class A(Generic[out_T]):
@classmethod
def f(cls, t: out_T) -> None:
print("hi")
```

The issue provides only a Python reproduction using Generic, TypeVar, and @classmethod; start by checking how basedmypy currently analyzes that example and what diagnostic it produces. No files or tests are named, so identify the relevant type-checking entry point and add coverage showing the example is accepted without regressing related variance checks.

Written by the indexing model from the issue text.