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/pyexpat/__init__.pyi

@@ -1,8 +1,7 @@
 import pyexpat.errors as errors
 import pyexpat.model as model
-from typing import Any, Callable, Dict, List, Optional, Text, Tuple, Union
-
 from _typeshed import SupportsRead
+from typing import Any, Callable, Dict, List, Optional, Text, Tuple, Union
 
 EXPAT_VERSION: str  # undocumented
 version_info: Tuple[int, int, int]  # undocumented