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/pathlib.pyi

@@ -1,10 +1,9 @@
 import os
 import sys
+from _typeshed import OpenBinaryMode, OpenTextMode
 from types import TracebackType
 from typing import IO, Any, BinaryIO, Generator, List, Optional, Sequence, TextIO, Tuple, Type, TypeVar, Union, overload
 
-from _typeshed import OpenBinaryMode, OpenTextMode
-
 _P = TypeVar("_P", bound=PurePath)
 
 if sys.version_info >= (3, 6):