Store bytes in memcached session, but always return native strings

[webknjaz]

Fix #1453

[jaraco]

ntob and tonative are deprecated constructs, so probably shouldn't be used here. An the very least, I think the change to _set_id is no better than the implementation that's in place now. Why ensure it's unicode just to convert it to bytes?

I can see the value in the change to _get_id as it provides backward compatibility for older versions of python-memcached on Python 2.

Personally, I'd be inclined to drop support for older versions of python-memcachd, and remove all of this workaround code altogether. What do you think?

[webknjaz]

Well, I think to do so we need to add dependency restriction.
I suggest adding extras to setup.py.
It would be useful for end-users as well. If they want to use memcached backed sessions they'll just specify (e.g.) cherrypy[memcached-sessions] and won't need to track memcached client version themselves.

[webknjaz]

@jaraco I think now it's OK to merge it

[jaraco]

Looks good to me. Can you add a CHANGES.rst entry as well?

[webknjaz]

Sure, done. Merging PR :)