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It can be done by copying back_bsddb.py into back_bsddb3.py
and systematically s/bsddb/bsddb3/
It works! It is kind of kludgy, because does not use bsddb3
locking system etc., but it is simplest way. And because
default bsddb is often buggy (much more buggy than
db-3.3.11 - I tried to load it with large data set - it 
misbehaves) it is recommended.