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/json/__init__.pyi

@@ -1,7 +1,6 @@
 import sys
-from typing import IO, Any, Callable, Dict, List, Optional, Tuple, Type, Union
-
 from _typeshed import SupportsRead
+from typing import IO, Any, Callable, Dict, List, Optional, Tuple, Type, Union
 
 from .decoder import JSONDecodeError as JSONDecodeError, JSONDecoder as JSONDecoder
 from .encoder import JSONEncoder as JSONEncoder