So, first, I guess you should open your database using parentheses:
db = bsddb.btopen('data.db','c')
Keep in mind that Berkeley's pattern is key -> value, where both key and value are string objects (not unicode). The best way in your case would be to use:
db[str(username)] = json.dumps(notes)
since your notes are compatible with the json syntax.
However, this is not a very good choice, say, if you want to query only usernames' comments. You should use a relational database, such as sqlite, which is also built-in in Python.