Feature: support Python 3.7+ with typing extensions (interested in TypedDict)

[pavelfeldman]

I've seen following here: python/typeshed#3500

import sys

if sys.version_info >= (3, 8):
    from typing import TypedDict, Literal, overload  # pylint: disable=no-name-in-module
else:
    from typing_extensions import TypedDict, Literal, overload