Remove third_party/3/enum.py (#2563)

All Python 3 versions supported by typeshed (3.4+) have enum as part
of the standard library.

Make the third-party Python 2 version consistent with the Python 3 version.

third_party/2/enum.pyi

@@ -1,4 +1,6 @@
-from typing import List, Any, TypeVar, Union, Iterable, Iterator, TypeVar, Generic, Type, Sized, Reversible, Container, Mapping
+# NB: third_party/2/enum.pyi and stdlib/3.4/enum.pyi must remain consistent!
+import sys
+from typing import Any, Iterator, List, Mapping, Type, TypeVar, Union
 from abc import ABCMeta
 
 _T = TypeVar('_T')
@@ -34,3 +36,30 @@ class IntEnum(int, Enum):
     value = ...  # type: int
 
 def unique(enumeration: _S) -> _S: ...
+
+if sys.version_info >= (3, 6):
+    _auto_null = ...  # type: Any
+
+    # subclassing IntFlag so it picks up all implemented base functions, best modeling behavior of enum.auto()
+    class auto(IntFlag):
+        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: ...
+
+    # The `type: ignore` comment is needed because mypy considers the type
+    # signatures of several methods defined in int and Flag to be incompatible.
+    class IntFlag(int, Flag):  # type: ignore
+        def __or__(self: _T, other: Union[int, _T]) -> _T: ...
+        def __and__(self: _T, other: Union[int, _T]) -> _T: ...
+        def __xor__(self: _T, other: Union[int, _T]) -> _T: ...
+        __ror__ = __or__
+        __rand__ = __and__
+        __rxor__ = __xor__