Add Flag, IntFlag and auto to enum stubs

Fixes python/mypy#2609

stdlib/3.4/enum.pyi

@@ -1,4 +1,9 @@
-from typing import List, Any, TypeVar
+# FIXME: Stub incomplete, ommissions include:
+# * the metaclass
+# * _sunder_ methods with their transformations
+
+import sys
+from typing import List, Any, TypeVar, Union
 
 class Enum:
     def __new__(cls, value: Any) -> None: ...
@@ -18,3 +23,28 @@ class IntEnum(int, Enum):
 _T = TypeVar('_T')
 
 def unique(enumeration: _T) -> _T: ...
+
+if sys.version_info >= (3, 6):
+    _auto_null = ...  # type: Any
+
+    class auto:
+        value = ...  # type: Any
+
+    class Flag(Enum):
+        def __contains__(self: _T, other: _T) -> bool: ...
+        def __repr__(self) -> str: ...
+        def __str__(self) -> str: ...
+        def __bool__(self) -> bool: ...
+        def __or__(self: _T, other: _T) -> _T: ...
+        def __and__(self: _T, other: _T) -> _T: ...
+        def __xor__(self: _T, other: _T) -> _T: ...
+        def __invert__(self: _T) -> _T: ...
+
+    # All `type: ignore` comments below due to IntFlag making the function signatures more permissive.
+    class IntFlag(int, Flag):  # type: ignore
+        def __or__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore
+        def __and__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore
+        def __xor__(self: _T, other: Union[int, _T]) -> _T: ...  # type: ignore
+        __ror__ = __or__
+        __rand__ = __and__
+        __rxor__ = __xor__