Backwards compatibility with Python 2.7 and unicode?

[dr-rodriguez]

Saw this line and realized it would fail on python 2.7 if the user provides a unicode rather than a string:

astrodbkit/astrodbkit/astrodb.py

Line 89 in c7cf049

if isinstance(dbpath, str):

If we add six to the list of requirements, we could do something like:
if isinstance(dbpath, six.string_types):
to treat both str and unicode the same way in both python 2 and 3. However, we can also get away without updating this at the moment.