[red-knot] Type narrowing for isinstance checks

[sharkdp]

Summary

Add type narrowing for isinstance(object, classinfo) checks:

x = 1 if flag else "a"

if isinstance(x, int):
    reveal_type(x)  # revealed: Literal[1]

To do:

• Should we emit a diagnostic if we detect that arguments to the isinstance call are incorrect? Or would that happen anyway in type inference, because we would check isinstance calls against this typeshed signature:

  def isinstance(obj: object, class_or_tuple: _ClassInfo, /) -> bool: ...

  => would happen in type inference/checking.
• Add support for PEP 604 union types on the right-hand side. Supporting str | int is different from supporting (str, int): For the latter, we can simply use the inferred type (tuple[type[str], type[int]]) and generate a constraint from it. However, if we attempt to do the same for the former, we would get builtin.UnionType (or @Todo, currently). So we probably need to walk the AST and extract the necessary information from there, similar to how we would generate a type annotation from the AST of str | int. => deferred for now

closes #13893

Test Plan

New Markdown-based tests in narrow/isinstance.md.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[carljm]

Looks great! Just one nit.

[AlexWaygood]

Looks great!

[carljm]

🎉