More python3 compatibility
@jerome, I was trying to make zodbtools work with Python3 and along that road picked some bits of your work from !12. At present the migration to Python3 is not complete, and even though now I have the answer to how handle strings in both python2/3 in compatible and reasonable way (I can share details if you are interested), I have to put that work on hold for some time and use https://pypi.org/project/pep3134 directly in wcfs tests, since getting all string details right, even after figuring on how to do it, will take time. Anyway the bits presented here should be ready for master and could be merged now. Could you please have a look?