Add FrozenSet to the typing stub files (#474)

Author: Vgr255

stdlib/2.7/typing.pyi

@@ -145,6 +145,8 @@ class AbstractSet(Sized, Iterable[_T_co], Container[_T_co], Generic[_T_co]):
     # TODO: argument can be any container?
     def isdisjoint(self, s: AbstractSet[Any]) -> bool: ...
 
+class FrozenSet(AbstractSet[_T], Generic[_T]): ...
+
 class MutableSet(AbstractSet[_T], Generic[_T]):
     @abstractmethod
     def add(self, x: _T) -> None: ...

stdlib/3/typing.pyi

@@ -189,6 +189,8 @@ class AbstractSet(Iterable[_T_co], Container[_T_co], Sized, Generic[_T_co]):
     # TODO: Argument can be a more general ABC?
     def isdisjoint(self, s: AbstractSet[Any]) -> bool: ...
 
+class FrozenSet(AbstractSet[_T], Generic[_T]): ...
+
 class MutableSet(AbstractSet[_T], Generic[_T]):
     @abstractmethod
     def add(self, x: _T) -> None: ...