`list.copy` should return a non-invariant value

```py
a = [1, 2, 3]
b: list[object] = a.copy() # error: invariance, expected no error
```

```py
class list[T]:
def copy[R >: T]() -> list[R]: ...
```
(which doesn't exist :()

No implementation file, test, or entry point is named. Reproduce the assignment example and inspect how list.copy() is currently typed; determine a supported way to make the result assignable to list[object]. Done means the example type-checks without an invariance error.

Written by the indexing model from the issue text.