Self is not rejected when used as a PEP 695 type parameter bound

Bug Report

Self is only valid in the specific locations listed in PEP 673 and Mypy already rejects several invalid locations.
See: https://github.com/python/mypy/blob/ae39cdb2fba8908eeff6ec8d6b88879c62495fcc/test-data/unit/check-selftype.test#L1480

It seems one location wasn't covered: using Self as the bound (or constraint) of a PEP 695 type parameter.

To Reproduce:

from typing import Self
class Foo:
      def fails[T: Self](self: T) -> None: pass

Expected Behavior

Mypy should reject Self here with an error.

From a quick look at other type checkers: pyright and ty treat it as an invalid/generic TypeVar bound (TypeVar upper bound cannot be generic), while zuban rejects it with the same message mypy already uses elsewhere,
Self type is only allowed in annotations within class definition.

Actual Behavior

No error is reported.

Your Environment

Mypy version: https://github.com/python/mypy/commit/ae39cdb2fba8908eeff6ec8d6b88879c62495fcc
Python version: 3.12

TLDR; originally spotted while working on #21928 See: https://github.com/python/mypy/pull/21928#issuecomment-5557910054

Inizia da test-data/unit/check-selftype.test intorno alla riga 1480 e riproduci l’esempio PEP 695 dell’issue. Segui la validazione esistente di Self usata per le altre posizioni non valide, quindi aggiungi la copertura per un bound o un vincolo di un parametro di tipo. Il lavoro è completo quando mypy segnala un errore per l’esempio, mentre i test Self esistenti continuano a passare.