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/io.pyi

@@ -1,12 +1,11 @@
 import builtins
 import codecs
 import sys
+from _typeshed import ReadableBuffer, WriteableBuffer
 from mmap import mmap
 from types import TracebackType
 from typing import IO, Any, BinaryIO, Callable, Iterable, Iterator, List, Optional, TextIO, Tuple, Type, TypeVar, Union
 
-from _typeshed import ReadableBuffer, WriteableBuffer
-
 DEFAULT_BUFFER_SIZE: int
 
 SEEK_SET: int