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/mailbox.pyi

@@ -1,4 +1,5 @@
 import email
+from _typeshed import AnyPath
 from types import TracebackType
 from typing import (
     IO,
@@ -21,11 +22,8 @@ from typing import (
     Union,
     overload,
 )
-
 from typing_extensions import Literal
 
-from _typeshed import AnyPath
-
 _T = TypeVar("_T")
 _MessageType = TypeVar("_MessageType", bound=Message)
 _MessageData = Union[email.message.Message, bytes, str, IO[str], IO[bytes]]