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/logging/__init__.pyi

@@ -1,5 +1,6 @@
 import sys
 import threading
+from _typeshed import StrPath
 from string import Template
 from time import struct_time
 from types import FrameType, TracebackType
@@ -20,8 +21,6 @@ from typing import (
     overload,
 )
 
-from _typeshed import StrPath
-
 _SysExcInfoType = Union[Tuple[type, BaseException, Optional[TracebackType]], Tuple[None, None, None]]
 if sys.version_info >= (3, 5):
     _ExcInfoType = Union[None, bool, _SysExcInfoType, BaseException]

stdlib/2and3/logging/config.pyi

@@ -1,9 +1,8 @@
 import sys
+from _typeshed import AnyPath, StrPath
 from threading import Thread
 from typing import IO, Any, Callable, Dict, Optional, Union
 
-from _typeshed import AnyPath, StrPath
-
 if sys.version_info >= (3,):
     from configparser import RawConfigParser
 else:

stdlib/2and3/logging/handlers.pyi

@@ -1,12 +1,11 @@
 import datetime
 import ssl
 import sys
+from _typeshed import StrPath
 from logging import FileHandler, Handler, LogRecord
 from socket import SocketType
 from typing import Any, Callable, Dict, List, Optional, Tuple, Union, overload
 
-from _typeshed import StrPath
-
 if sys.version_info >= (3, 7):
     from queue import SimpleQueue, Queue
 elif sys.version_info >= (3,):