go: Proper support for py3-generated data and py3

• Add robust support for data generated by python3 based on ogórek StrictUnicode mode.
• Test handling of all ZODB kinds of data we care about: py2/pickle{1,2,3} and py3/pickle3, fix issues discovered along the way.
• Fix interoperability with py3 programs: for example previously index, that FileStorage/go saves, was not readable on py3 side.
• Switch encoding protocol for pickles, that ZODB/go and other go places emit, from 2 to 3.
• EDIT 20240820: Switch unpickling to work in PyDict=y mode.
• EDIT 20240820: Add tests for zodb.Map and zodb.List .
• minor fixes.

Robust support for py3-generated data is needed for Wendelin.core .

This work depends on recent zodbtools update from https://lab.nexedi.com/nexedi/zodbtools/-/merge_requests/29.

/cc @jerome, @levin.zimmermann, @vnmabus