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/subprocess.pyi

@@ -1,11 +1,9 @@
 import sys
+from _typeshed import AnyPath
 from types import TracebackType
 from typing import IO, Any, AnyStr, Callable, Generic, Mapping, Optional, Sequence, Tuple, Type, TypeVar, Union, overload
-
 from typing_extensions import Literal
 
-from _typeshed import AnyPath
-
 # We prefer to annotate inputs to methods (eg subprocess.check_call) with these
 # union types.
 # For outputs we use laborious literal based overloads to try to determine