python/py3: fix type stubs for enums
Summary:
with
```
class Priority(Enum):
...
value: int
```
pyre will think `value` is another member of the `Priority` enum, instead of a property of all `Priority` enums, and somehow ignore the `: int` hint
it might be a pyre bug, but there is a workaround: we can replace it with
```
class Priority(Enum):
...
property
def value(self) -> int: ...
```
which will clear the confusion
Reviewed By: ParalaxRus
Differential Revision:
D40392028
fbshipit-source-id:
005d408c24a257a787aff4e2c2c6af23be50da08