PYI019 causes false positives on .py files if you support Python <=3.10

[AlexWaygood]

I just tried enabling ruff's PYI rules on typeshed-stats, a hobby project of mine, and ran into a false positive with PYI019. I have the following class in the project:

from enum import Enum
from typing import TypeVar

_NiceReprEnumSelf = TypeVar("_NiceReprEnumSelf", bound="_NiceReprEnum")

class _NiceReprEnum(Enum):
    """Base class for several public-API enums in this package."""

    def __new__(cls: type[_NiceReprEnumSelf], doc: str) -> _NiceReprEnumSelf:
        assert isinstance(doc, str)
        member = object.__new__(cls)
        member._value_ = member.__doc__ = doc
        return member

PYI019 tells me off about this, saying:

src\typeshed_stats\gather.py:88:60: PYI019 Methods like `__new__` should return `typing.Self` instead of a custom `TypeVar`

But I still support Python 3.10 (typing.Self is new in Python 3.11), and don't have typing_extensions as a dependency, so this violation isn't actionable.

(This isn't an issue for .pyi files, as type checkers think typing_extensions is part of the stdlib due to some white lies we tell them in typeshed. That means Self can always be imported from typing_extensions in a .pyi file, even if the stubs package doesn't explicitly declare a dependency on typing_extensions.)