Port zodbtools to py3
Penultimate patch needs bstr from pygolang to work ok (see https://lab.nexedi.com/kirr/pygolang/commit/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:
- https://lab.nexedi.com/nexedi/zodbtools/merge_requests/8#note_73726
- https://lab.nexedi.com/nexedi/zodbtools/merge_requests/12
- conversation from https://lab.nexedi.com/nexedi/zodbtools/merge_requests/13#note_81553 to https://lab.nexedi.com/nexedi/zodbtools/merge_requests/13#note_81874
- https://lab.nexedi.com/nexedi/zodbtools/merge_requests/19#note_129023
- https://lab.nexedi.com/kirr/zodbtools/commit/42799cf6#note_166403
/cc @jerome