Open a dbm.sqlite3 database and return the dbm object.
The 'filename' parameter is the name of the database file.
The optional 'flag' parameter can be one of ...:
'r' (default): open an existing database for read only access
'w': open an existing database for read/write access
'c': create a database if it does not exist; open for read/write access
'n': always create a new, empty database; open for read/write access
The optional 'mode' parameter is the Unix file access mode of the database;
only used when creating a new database. Default: 0o666.