Args for "flexible callable" experimental mypy feature. (#793)

This is the typeshed for the constructors for the Arg types that we'll now be
able to pass to Callable.  They really just return their type arguments.

Author: sixolet

third_party/2and3/mypy_extensions.pyi

@@ -1,10 +1,17 @@
-from typing import Dict, Type, TypeVar, Union
+from typing import Dict, Type, TypeVar, Optional, Union
 
 _T = TypeVar('_T')
 
 
 def TypedDict(typename: str, fields: Dict[str, Type[_T]]) -> Type[dict]: ...
 
+def Arg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def DefaultArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def NamedArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def DefaultNamedArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def VarArg(type: _T = ...) -> _T: ...
+def KwArg(type: _T = ...) -> _T: ...
+
 # Return type that indicates a function does not return.
 # This type is equivalent to the None type, but the no-op Union is necessary to
 # distinguish the None type from the None value.