adjust isort config (#4290)

Fixes #4288.

- Default imports to THIRD_PARTY, so in effect we merge the FIRST_PARTY and THIRD_PARTY stubs. This means import order is no longer affected by whether typing_extensions is installed locally.
- Treat typing_extensions, _typeshed and some others as standard library modules.

Note that isort master is very different from the latest release; we'll have to do something
different if and when the next isort release comes out.

third_party/3/waitress/trigger.pyi

@@ -2,7 +2,6 @@ import sys
 from socket import SocketType
 from threading import Lock
 from typing import Callable, Mapping, Optional
-
 from typing_extensions import Literal
 
 from . import wasyncore as wasyncore