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/3/hashlib.pyi

@@ -1,9 +1,8 @@
 # Stubs for hashlib
 
 import sys
-from typing import AbstractSet, Optional, Union
-
 from _typeshed import ReadableBuffer
+from typing import AbstractSet, Optional, Union
 
 class _Hash(object):
     digest_size: int