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/json.pyi

@@ -1,6 +1,5 @@
-from typing import IO, Any, Callable, Dict, List, Optional, Text, Tuple, Type, Union
-
 from _typeshed import SupportsRead
+from typing import IO, Any, Callable, Dict, List, Optional, Text, Tuple, Type, Union
 
 class JSONDecodeError(ValueError):
     def dumps(self, obj: Any) -> str: ...