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/venv/__init__.pyi

@@ -1,9 +1,8 @@
 import sys
+from _typeshed import AnyPath
 from types import SimpleNamespace
 from typing import Optional, Sequence
 
-from _typeshed import AnyPath
-
 class EnvBuilder:
     system_site_packages: bool
     clear: bool