Py3 cleanups: remove unwanted __slots__
Py3 cleanups: remove unwanted __slots__

__slots__ cannot be overriden by property getters (silent in py2, error
in py3)

Signed-off-by: Yves Bastide <3b1fe340dba76bf37270abad774f327f50b5e1d8@botify.com>

botify-labs/simpleflow,botify-labs/simpleflow