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/fcntl.pyi

@@ -1,10 +1,8 @@
+from _typeshed import FileDescriptorLike
 from array import array
 from typing import Any, Union, overload
-
 from typing_extensions import Literal
 
-from _typeshed import FileDescriptorLike
-
 FASYNC: int
 FD_CLOEXEC: int
 DN_ACCESS: int