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/pydoc.pyi

@@ -1,4 +1,5 @@
 import sys
+from _typeshed import SupportsWrite
 from types import FunctionType, MethodType, ModuleType, TracebackType
 from typing import (
     IO,
@@ -18,8 +19,6 @@ from typing import (
     Union,
 )
 
-from _typeshed import SupportsWrite
-
 if sys.version_info >= (3,):
     from reprlib import Repr
 else: