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/asyncio/base_events.pyi

@@ -1,5 +1,6 @@
 import ssl
 import sys
+from _typeshed import FileDescriptorLike
 from abc import ABCMeta
 from asyncio.events import AbstractEventLoop, AbstractServer, Handle, TimerHandle
 from asyncio.futures import Future
@@ -8,11 +9,8 @@ from asyncio.tasks import Task
 from asyncio.transports import BaseTransport
 from socket import _Address, _RetAddress, socket
 from typing import IO, Any, Awaitable, Callable, Dict, Generator, List, Optional, Sequence, Tuple, TypeVar, Union, overload
-
 from typing_extensions import Literal
 
-from _typeshed import FileDescriptorLike
-
 if sys.version_info >= (3, 7):
     from contextvars import Context
 

stdlib/3/asyncio/base_futures.pyi

@@ -1,6 +1,5 @@
 import contextvars
 from typing import Callable, List, Sequence, Tuple
-
 from typing_extensions import Literal
 
 from . import futures

stdlib/3/asyncio/base_tasks.pyi

@@ -1,8 +1,7 @@
+from _typeshed import AnyPath
 from types import FrameType
 from typing import List, Optional
 
-from _typeshed import AnyPath
-
 from . import tasks
 
 def _task_repr_info(task: tasks.Task) -> List[str]: ...  # undocumented

stdlib/3/asyncio/events.pyi

@@ -1,5 +1,6 @@
 import ssl
 import sys
+from _typeshed import FileDescriptorLike
 from abc import ABCMeta, abstractmethod
 from asyncio.futures import Future
 from asyncio.protocols import BaseProtocol
@@ -9,8 +10,6 @@ from asyncio.unix_events import AbstractChildWatcher
 from socket import _Address, _RetAddress, socket
 from typing import IO, Any, Awaitable, Callable, Dict, Generator, List, Optional, Sequence, Tuple, TypeVar, Union, overload
 
-from _typeshed import FileDescriptorLike
-
 if sys.version_info >= (3, 7):
     from contextvars import Context
 

stdlib/3/asyncio/proactor_events.pyi

@@ -1,6 +1,5 @@
 from socket import socket
 from typing import Any, Mapping, Optional
-
 from typing_extensions import Literal
 
 from . import base_events, constants, events, futures, streams, transports

stdlib/3/asyncio/sslproto.pyi

@@ -1,7 +1,6 @@
 import ssl
 import sys
 from typing import Any, Callable, ClassVar, Deque, Dict, List, Optional, Tuple
-
 from typing_extensions import Literal
 
 from . import constants, events, futures, protocols, transports

stdlib/3/asyncio/tasks.pyi

@@ -17,7 +17,6 @@ from typing import (
     Union,
     overload,
 )
-
 from typing_extensions import Literal
 
 from .events import AbstractEventLoop