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/socket.pyi

@@ -16,7 +16,6 @@ CPython C source: https://github.com/python/cpython/blob/master/Modules/socketmo
 #   adapted for Python 2.7 by Michal Pokorny
 import sys
 from typing import Any, BinaryIO, Iterable, List, Optional, Text, TextIO, Tuple, TypeVar, Union, overload
-
 from typing_extensions import Literal
 
 # ----- Constants -----