Custom properties can be specified by passing them to a STIX object
constructor in the 'custom_properties' argument, or with the
'allow_custom' argument set to True, which will add any unrecognized
keyword arguments as properties on the object. The 'allow_custom'
argument can also be used with the parse() and parse_observable()
functions.
An error is now raised when attempting to parse objects without a 'type'
property, such as external references, kill chain phases, and granular
markings. The object which contains them is what should be parsed, not
these objects themselves.
Emmanuelle Vargas-Gonzalez