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/traceback.pyi

@@ -1,9 +1,8 @@
 import sys
+from _typeshed import SupportsWrite
 from types import FrameType, TracebackType
 from typing import IO, Any, Dict, Generator, Iterable, Iterator, List, Mapping, Optional, Tuple, Type
 
-from _typeshed import SupportsWrite
-
 _PT = Tuple[str, int, str, Optional[str]]
 
 def print_tb(tb: Optional[TracebackType], limit: Optional[int] = ..., file: Optional[IO[str]] = ...) -> None: ...