Fixes MemcachedSessionInterface to work with Python 3

MemcachedSessionInterface checks if the key is unicode, and if so it encodes it to a string. 

In python 3 there is no longer a unicode class, but instead the bytes class. This update checks the version of python and if it's 2, it behaves as it did before, if it's 3, it checks to see if the key is a bytes object, and if so decodes it to a string object.

I ran test_session.py in both python 2 and python 3 and the tests passed.