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/trace.pyi

@@ -1,7 +1,6 @@
 import types
-from typing import Any, Callable, Mapping, Optional, Sequence, Tuple, TypeVar, Union
-
 from _typeshed import StrPath
+from typing import Any, Callable, Mapping, Optional, Sequence, Tuple, TypeVar, Union
 
 _T = TypeVar("_T")
 _localtrace = Callable[[types.FrameType, str, Any], Callable[..., Any]]