Repair pickling for Properties object
Fixed bug whereby the ``__getstate__`` / ``__setstate__``
methods for sqlalchemy.util.Properties were
non-working due to the transition in the 1.0 series to ``__slots__``.
The issue potentially impacted some third-party applications.
Pull request courtesy Pieter Mulder.
Fixes: #3728
Change-Id: I01ebd425bbfe145747fea2edd0d2d412c74fd84d
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/286
(cherry picked from commit
cab57e9bab04fbdea44690c08dff379a29eaab32)