[red-knot] GenericAlias instances as a base class (#17575)
## Summary
We currently emit a diagnostic for code like the following:
```py
from typing import Any
# error: Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)
class C(tuple[Any, ...]): ...
```
The changeset here silences this diagnostic by recognizing instances of
`GenericAlias` in `ClassBase::try_from_type`, and inferring a `@Todo`
type for them. This is a change in preparation for #17557, because `C`
previously had `Unknown` in its MRO …
```py
reveal_type(C.__mro__) # tuple[Literal[C], Unknown, Literal[object]]
```
… which would cause us to think that `C` is assignable to everything.
The changeset also removes some false positive `invalid-base`
diagnostics across the ecosystem.
## Test Plan
Updated Markdown tests.
