Penultimate patch needs bstr from pygolang to work ok (see kirr/pygolang@c9648c44), but it won't hurt if we merge this without waiting for pygolang bits because without bstr zodbtools continues to work ok on py2, and it will be py3 mode which will not work fully ok.

Previous discussions and py3 porting attempts:

• !8 (comment 73726)
• !12 (closed)
• conversation from !13 (comment 81553) to !13 (comment 81874)
• !19 (comment 129023)
• kirr/zodbtools@42799cf6 (comment 166403)

/cc @jerome