add (overwrite with) mypy stubs, if available

stdlib/3.4/enum.pyi

@@ -0,0 +1,19 @@
+from typing import List, Any, TypeVar
+
+class Enum:
+    def __new__(cls, value: Any) -> None: ...
+    def __repr__(self) -> str: ...
+    def __str__(self) -> str: ...
+    def __dir__(self) -> List[str]: ...
+    def __format__(self, format_spec: str) -> str: ...
+    def __hash__(self) -> Any: ...
+    def __reduce_ex__(self, proto: Any) -> Any: ...
+
+    name = ''
+    value = None  # type: Any
+
+class IntEnum(int, Enum): ...
+
+_T = TypeVar('_T')
+
+def unique(enumeration: _T) -> _T: ...