Fixed being unable to access class attributes on a NamedTuple

[AWhetter]

Description

In the case of an assignment to a NamedTuple without a type, the attribute becomes a class attribute.

>>> from typing import NamedTuple
>>> class MyTuple(NamedTuple):
...     CLASS_ATTR = "class_attr"
...     instance_attr: str
...     default_attr: str = "default"
... 
>>> my_tuple = MyTuple("instance")
>>> my_tuple
MyTuple(instance_attr='instance', default_attr='default')
>>> my_tuple.CLASS_ATTR
'class_attr'
>>> MyTuple.CLASS_ATTR
'class_attr'

This pull request adds the copying of class attributes to subclasses of NamedTuple .

Type of Changes

	Type
✓	🐛 Bug fix
	✨ New feature
	🔨 Refactoring
	📜 Docs

Related Issue

Close pylint-dev/pylint#1628

[PCManticore]

Thank you! The AppVeyor failure is unrelated, let's merge this.