[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.

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -81,13 +81,11 @@ python-version = "3.9"
 ```
 
 ```py
-# TODO: `tuple[int, str]` is a valid base (generics)
-# error: [invalid-base] "Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
 class A(tuple[int, str]): ...
 
 # Runtime value: `(A, tuple, object)`
 # TODO: Generics
-reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Unknown, Literal[object]]
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], @Todo(GenericAlias instance), Literal[object]]
 ```
 
 ## `typing.Tuple`