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.

stdlib/3/http/__init__.pyi

@@ -1,6 +1,5 @@
 import sys
 from enum import IntEnum
-
 from typing_extensions import Literal
 
 class HTTPStatus(IntEnum):