Open the database file filename and return a corresponding object.
The optional flag argument can be
'r' to open an existing database for reading only,
'w' to open an existing database for reading and writing,
'c' to create the database if it doesn't exist, or
'n', which will always create a new empty database. If not
specified, the default value is 'r'.
The optional mode argument is the Unix mode of the file, used
only when the database has to be created. It defaults to octal
0666 (and will be modified by the prevailing umask).