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/gzip.pyi

@@ -1,11 +1,9 @@
 import _compression
 import sys
 import zlib
-from typing import IO, Optional, TextIO, Union, overload
-
-from typing_extensions import Literal
-
 from _typeshed import AnyPath, ReadableBuffer
+from typing import IO, Optional, TextIO, Union, overload
+from typing_extensions import Literal
 
 _OpenBinaryMode = Literal["r", "rb", "a", "ab", "w", "wb", "x", "xb"]
 _OpenTextMode = Literal["rt", "at", "wt", "xt"]