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/lzma.pyi

@@ -1,9 +1,7 @@
 import io
-from typing import IO, Any, Mapping, Optional, Sequence, TextIO, TypeVar, Union, overload
-
-from typing_extensions import Literal
-
 from _typeshed import AnyPath, ReadableBuffer
+from typing import IO, Any, Mapping, Optional, Sequence, TextIO, TypeVar, Union, overload
+from typing_extensions import Literal
 
 _OpenBinaryWritingMode = Literal["w", "wb", "x", "xb", "a", "ab"]
 _OpenTextWritingMode = Literal["wt", "xt", "at"]