Opens a calendar database and returns a new CalendarDb object.
If filename is
None, the default database is opened.
If filename is given, it should contain drive letter, colon and file's name, but no absolute path.
mode can be:
None: Opens an existing calendar database.
'c': Opens an existing calendar database, or creates it if it doesn't exist.
'n': Creates a new, empty calendar database. If filename exists, the previous contents are erased.
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