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
- conversation from !13 (comment 81553) to !13 (comment 81874)
- !19 (comment 129023)
- kirr/zodbtools@42799cf6 (comment 166403)

/reviewed-by @jerome
/reviewed-on !23