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/2/os/__init__.pyi

@@ -1,4 +1,5 @@
 import sys
+from _typeshed import AnyPath
 from builtins import OSError as error
 from io import TextIOWrapper as _TextIOWrapper
 from posix import listdir as listdir, stat_result as stat_result  # TODO: use this, see https://github.com/python/mypy/issues/3078
@@ -25,8 +26,6 @@ from typing import (
     overload,
 )
 
-from _typeshed import AnyPath
-
 from . import path as path
 
 # We need to use something from path, or flake8 and pytype get unhappy

stdlib/2/os/path.pyi

@@ -1,11 +1,10 @@
 # NB: path.pyi and stdlib/2 and stdlib/3 must remain consistent!
 import os
 import sys
+from _typeshed import AnyPath, BytesPath, StrPath
 from genericpath import exists as exists
 from typing import Any, AnyStr, Callable, List, Optional, Sequence, Text, Tuple, TypeVar, overload
 
-from _typeshed import AnyPath, BytesPath, StrPath
-
 _T = TypeVar("_T")
 
 if sys.version_info >= (3, 6):