Pass most conformance tests for Zuban

[davidhalter]

This latest update passes most conformance tests for Zuban. I simplified the flags for zuban check and it also passes 2 more tests for Pyrefly.

I have made several small changes to the tests. These should mostly be uncontroversial:

• Made it possible to report unreachable after a function that returns Never.
• Removed a redundant import that Zuban reported
• A method can now be inferred as SelfTypeVar = TypeVar("SelfTypeVar", bound=Foo7[int]); Callable[[SelfTypevar], SelfTypeVar] instead of Callable[[Foo7[int]], Foo7[int]]. I personally believe this should be how we infer things, since it's more precise but I don't think we have to mandate this.
• I allowed type checkers to return list[Any] instead of Any in an overload, if there was no match, because both of the overloads return a list and it is therefore just a slightly more precise type.
• I added an additional optional error for taking a TypeVar of an outer generic class (in an __init__). There is already an error for the outer attribute x, but I feel like this error should be fine.

There remain 4 non-passing tests for Zuban, all of which I'm not sure the tests are doing the right thing. I will be discussing these in the typing discourse.