Previously we were unpickling dicts into map[any]any and it was making a
difference from which Python version the dict was saved: if it was saved
from py2 with str keys, then the dict needed to be accessed with ByteString
keys, while if it was saved from py3 with str keys, the dict needed to
be accessed with string keys. This peculiarity is one of the reasons why
Map test currently fails on py3_pickle3 - because there - both in the
test and inside Map.PySetState, dictionaries are accessed via ByteString
keys which works only wrt py2-saved data.

-> Fix it by switching the unpickler to work in PyDict=y mode(*) which
results in dicts unpickled as ogórek.Dict instead of map[any]any
with the ogórek.Dict considering ByteString to be equal to both string and Bytes
with the same underlying content. This allows programs to access Dict via
string/bytes keys following Python3 model, while still being able to handle
dictionaries generated from under Python2.

For ZODB/go this is change in behaviour exposed to outside. However
there is currently only one known ZODB/go user - WCFS in Wendelin.core -
and that user will be updated correspondingly as well.

The change fixes the following failure in go/zodb tests

    --- FAIL: TestPersistentMapListLoad/py3_pickle3 (0.70s)
        persistent_x_test.go:57: persistent.mapping.PersistentMapping(0000000000000000): activate: pysetstate: noone of ["data" "_container"] is present in state dict

but one more remains there which will be addressed in the next patch.

(*) see https://github.com/kisielk/og-rek/pull/75 for details.