Previously we were testing ZBlk loading on Go side only with data generated by
python2 and pickle protocol=2. However even on py2 there are more pickle
protocols that are in use, and also there is python3.

-> Modernize testdata/zblk_test_gen.py to use run_with_all_zodb_pickle_kinds
   that was recently added as part of nexedi/zodbtools@f9d36ba7
   and generate test data with both python2 and python3. It is handy to
   use py2py3-venv(*) to prepare python environment to do that.

   Adjust tests on Go side to verify how ZBlk is loaded for all generated zkinds.

py2_pickle1, py2_pickle2 and py2_pickle3 are handled well.
ZBlk test for py3_pickle3 currently fails with

        --- FAIL: TestZBlk/py3_pickle3 (0.01s)
    panic: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes [recovered]

and so is marked with "xfail".

We will fix tests for py3_pickle3 in follow-up patches.

(*) see nexedi/zodbtools@fac2f190