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/fileinput.pyi

@@ -1,7 +1,6 @@
 import sys
-from typing import IO, Any, AnyStr, Callable, Generic, Iterable, Iterator, Optional, Union
-
 from _typeshed import AnyPath
+from typing import IO, Any, AnyStr, Callable, Generic, Iterable, Iterator, Optional, Union
 
 if sys.version_info >= (3, 8):
     def input(