We need to make sure that an existing instance, set up with Python 2,
including UTF-8 data in the database not just plain ASCII, works
correctly when used with Python 3 - that the database content format is
properly compatible across Python versions - with every database back
end (the reverse, creating with Python 3 and using with Python 2, may be
less important, but is still desirable - if someone has problems
updating their instance to Python 3, they should be able to move back to
Python 2 without the stored data being incompatible).  anydbm may have
the highest risk of problems here, as it uses the Python marshal module
in storing data.