Removed unused import symbols from stdlib stubs and fixed a few other errors detected by pyright. I did a similar pass several months ago; these were introduced since then. (#4803)

Co-authored-by: Eric Traut <erictr@microsoft.com>

stdlib/3/concurrent/futures/_base.pyi

@@ -2,7 +2,6 @@ import sys
 import threading
 from abc import abstractmethod
 from logging import Logger
-from types import TracebackType
 from typing import (
     Any,
     Callable,