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/ssl.pyi

@@ -1,11 +1,9 @@
 import enum
 import socket
 import sys
-from typing import Any, Callable, ClassVar, Dict, Iterable, List, NamedTuple, Optional, Set, Text, Tuple, Type, Union, overload
-
-from typing_extensions import Literal
-
 from _typeshed import StrPath
+from typing import Any, Callable, ClassVar, Dict, Iterable, List, NamedTuple, Optional, Set, Text, Tuple, Type, Union, overload
+from typing_extensions import Literal
 
 _PCTRTT = Tuple[Tuple[str, str], ...]
 _PCTRTTT = Tuple[_PCTRTT, ...]