Support enum iteration (#1136)

* Support enum iteration.

Fixes python/mypy#2305.

* Make EnumMeta inherit from type, not ABCMeta.

Author: gvanrossum

stdlib/3.4/enum.pyi

@@ -1,12 +1,13 @@
-# FIXME: Stub incomplete, ommissions include:
-# * the metaclass
-# * _sunder_ methods with their transformations
-
 import sys
-from typing import List, Any, TypeVar, Union
+from typing import List, Any, TypeVar, Union, Iterable, Iterator, TypeVar, Generic, Type
+
+_T = TypeVar('_T', bound=Enum)
 
-class Enum:
-    def __new__(cls, value: Any) -> None: ...
+class EnumMeta(type, Iterable[Enum]):
+    def __iter__(self: Type[_T]) -> Iterator[_T]: ...  # type: ignore
+
+class Enum(metaclass=EnumMeta):
+    def __new__(cls: Type[_T], value: Any) -> _T: ...
     def __repr__(self) -> str: ...
     def __str__(self) -> str: ...
     def __dir__(self) -> List[str]: ...
@@ -20,8 +21,6 @@ class Enum:
 class IntEnum(int, Enum):
     value = ...  # type: int
 
-_T = TypeVar('_T')
-
 def unique(enumeration: _T) -> _T: ...
 
 if sys.version_info >= (3, 6):