Add TypedDict total argument

See python/mypy#3558 for context.

Author: JukkaL

third_party/2and3/mypy_extensions.pyi

@@ -2,7 +2,7 @@ from typing import Dict, Type, TypeVar, Optional, Union
 
 _T = TypeVar('_T')
 
-def TypedDict(typename: str, fields: Dict[str, Type[_T]]) -> Type[dict]: ...
+def TypedDict(typename: str, fields: Dict[str, Type[_T]], total: bool = ...) -> Type[dict]: ...
 
 def Arg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
 def DefaultArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...