stubgen: don't recurse forever on a class that refers back to itself

[Eljees]

Fixes #11989.

InspectionStubGenerator.generate_class_stub() walks the attributes of a class and recurses into every attribute whose value is a class defined in the same module. A class that exposes itself — C.C = C, which is what the issue's _socket.CAPI-style extension types do — therefore makes it recurse into itself until Python raises RecursionError.

The fix skips an attribute whose value is the class currently being generated or any class enclosing it. Such an attribute is a back reference, not a nested class, so nothing that was previously emitted is lost.

Note that comparing names (attr != class_name), as suggested in the issue, only covers half of the cases: when two classes in the same module expose each other (P.C = C; C.P = P) the names differ and the recursion is unchanged. The identity walk up ClassInfo.parent handles both; the branch already carried parent_class=class_info, so the chain was available.

Tests: two cases in mypy/test/teststubgen.py — a self-referential class and a mutually referential pair. Both raise RecursionError on master and pass with the change; the full teststubgen.py suite (367 tests) is green, and self-check, ruff and black are clean.