Switch to typing_extensions.Self for __new__ implementations

[bigfootjon]

I noticed that when running with https://pyrefly.org/ I get type errors on things like:

from graphql import GraphQLEnumType

def get_enum() -> GraphQLEnumType:
  return GraphQLEnumType()

This occurs because the __new__ function for GraphQLNamedType returns GraphQLNamedType (i.e. that exact class) instead of Self which automatically infers the actual class (e.g. GraphQLEnumType in this case).

I used the typing_extensions version because AIUI this project supports versions of python from before Self was introduced, and I also fixed the other __new__ function that returns a type while I was fixing the problem I was seeing.

[leszekhanusz]

In gql, we are importing Self like this:

import sys
if sys.version_info >= (3, 11):
    from typing import Self  # pragma: no cover
else:
    from typing_extensions import Self  # pragma: no cover

I think it makes it easier for the maintainer to know when we can avoid using typing_extensions when we remove the support for an old Python version.

[Cito]

Thank you @bigfootjon, this makes sense.

As @leszekhanusz said, this should be only imported conditionally, and the import should go to the TYPE_CHECKING block.

Usually I don't even check the sys.version, but just try to import from typing and then from type extensions if that fails - like here. But I add the Python version as comment, so that it can be searched and removed when no longer needed.