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/macpath.pyi

@@ -3,9 +3,8 @@
 
 import os
 import sys
-from typing import Any, AnyStr, Callable, List, Optional, Sequence, Text, Tuple, TypeVar, Union, overload
-
 from _typeshed import AnyPath, BytesPath, StrPath
+from typing import Any, AnyStr, Callable, List, Optional, Sequence, Text, Tuple, TypeVar, Union, overload
 
 if sys.version_info < (3, 8):
     _T = TypeVar("_T")