fix lifetime of return value in PyClassGuardMutSuper::as_super

[davidhewitt]

PyClassGuardMutSuper::as_super was incorrectly returning a super-guard with lifetime 'a decoupled from the &mut borrow of the guard. This allows the super-guard to exist after the .as_super() borrow ends, potentially leading to mutable aliasing issues.

cc @Icxolu - we missed this in #6104

(Credit to Codex security scanning for this discovery.)

[davidhewitt]

I'm torn whether to ship this in 0.29.1. In theory it could break builds, but it seems like anyone who would be broken by this potentially has issues anyway. (So I think my leaning is that it's better to ship it.)

I don't think it's bad enough to warrant yanking 0.29.0.

[Icxolu]

Yeah, in hindsight this is quite clearly wrong 🙈. I agree that we should ship in 0.29.1. If this breaks someone, that code is also clearly wrong. Also we have not yet really advertised this type much and not introduced a direct way to borrow a guard, only via extract, so I doubt there will be many uses already.