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/hmac.pyi

@@ -1,11 +1,10 @@
 # Stubs for hmac
 
 import sys
+from _typeshed import ReadableBuffer
 from types import ModuleType
 from typing import Any, AnyStr, Callable, Optional, Union, overload
 
-from _typeshed import ReadableBuffer
-
 _B = Union[bytes, bytearray]
 
 # TODO more precise type for object of hashlib