I would also be OK with something like

```
MGF1Enum = Enum("MGF1Enum", "MAX_LENGTH")
...
class MGF1(object):
    MAX_LENGTH = MGF1Enum.MAX_LENGTH
```

but then we need to depend on the `enum` backport package for 2.x too.

Something that avoids using actual values (like `-2` or `"MAX_LENGTH"`) for magical flag tokens is all I am after :)