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/lib2to3/pgen2/driver.pyi

@@ -1,10 +1,9 @@
+from _typeshed import StrPath
 from lib2to3.pgen2.grammar import Grammar
 from lib2to3.pytree import _NL, _Convert
 from logging import Logger
 from typing import IO, Any, Iterable, Optional, Text
 
-from _typeshed import StrPath
-
 class Driver:
     grammar: Grammar
     logger: Logger

stdlib/2and3/lib2to3/pgen2/grammar.pyi

@@ -1,6 +1,5 @@
-from typing import Dict, List, Optional, Text, Tuple, TypeVar
-
 from _typeshed import StrPath
+from typing import Dict, List, Optional, Text, Tuple, TypeVar
 
 _P = TypeVar("_P")
 _Label = Tuple[int, Optional[Text]]

stdlib/2and3/lib2to3/pgen2/pgen.pyi

@@ -1,9 +1,8 @@
+from _typeshed import StrPath
 from lib2to3.pgen2 import grammar
 from lib2to3.pgen2.tokenize import _TokenInfo
 from typing import IO, Any, Dict, Iterable, Iterator, List, NoReturn, Optional, Text, Tuple
 
-from _typeshed import StrPath
-
 class PgenGrammar(grammar.Grammar): ...
 
 class ParserGenerator: