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/2and3/xml/etree/ElementTree.pyi

@@ -1,4 +1,5 @@
 import sys
+from _typeshed import AnyPath, FileDescriptor, SupportsWrite
 from typing import (
     IO,
     Any,
@@ -19,11 +20,8 @@ from typing import (
     Union,
     overload,
 )
-
 from typing_extensions import Literal
 
-from _typeshed import AnyPath, FileDescriptor, SupportsWrite
-
 VERSION: str
 
 class ParseError(SyntaxError):

stdlib/2and3/xml/sax/saxutils.pyi

@@ -1,11 +1,10 @@
 import sys
+from _typeshed import SupportsWrite
 from codecs import StreamReaderWriter, StreamWriter
 from io import RawIOBase, TextIOBase
 from typing import Mapping, Optional, Text, TextIO, Union
 from xml.sax import handler, xmlreader
 
-from _typeshed import SupportsWrite
-
 def escape(data: Text, entities: Mapping[Text, Text] = ...) -> Text: ...
 def unescape(data: Text, entities: Mapping[Text, Text] = ...) -> Text: ...
 def quoteattr(data: Text, entities: Mapping[Text, Text] = ...) -> Text: ...