Adds support for NamedTuple subtyping

[sobolevn]

Closes #11160

[JukkaL]

I agree with Jelle that this is a bit questionable, since NamedTuple is not a real type, but I guess it could be useful in some cases. We can mark this as an experimental feature and see what users think about it.

Can you add a test case to pythoneval.test that uses the full stdlib stubs? Try accessing various attributes that might be useful in the test case. The new behavior depends on how NamedTuple is defined in typeshed.

Also, this would have to be documented since this is a non-standard extension.

[sobolevn]

@JukkaL I've added both pythoneval test and docs. Thanks a lot for the review! 👍

[sobolevn]

I need this change in my new semantic analyzer for namedtuples #11047 😞

[JukkaL]

Sorry for the late review! A few more minor comments.

[sobolevn]

Done! Docs and tests are improved 👍

[JukkaL]

Thanks, looks good now!

[sobolevn]

@JukkaL thank you 🎉 🎉