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/imaplib.pyi

@@ -4,7 +4,6 @@ import time
 from socket import socket as _socket
 from ssl import SSLContext, SSLSocket
 from typing import IO, Any, Callable, Dict, List, Optional, Pattern, Text, Tuple, Type, Union
-
 from typing_extensions import Literal
 
 # TODO: Commands should use their actual return types, not this type alias.