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/2and3/warnings.pyi

@@ -1,7 +1,6 @@
 import sys
 from types import ModuleType, TracebackType
 from typing import Any, List, NamedTuple, Optional, TextIO, Type, Union, overload
-
 from typing_extensions import Literal
 
 from _warnings import warn as warn, warn_explicit as warn_explicit