Version 4.0

The elevator and stepper are compliant with STIX 2.1 CS02

Python 3.5 is no longer supported.
All compatibility code to support Python 2.7 has been removed (e.g., the six package)

Major Changes

• Support Extension feature described in section 7.3 of the specification
• Support Incident as a specification SDO
• Use handle_missing_properties method to encapsulate most extension/custom functionality
• Handle related objects with no know STIX 2.1 relationship

Other Changes

• Handle ArchiveFile different for 2.0 vs 2.1
• email_message:message_id is only in 2.1
• Infer network-traffic protocol from extension type
• Handle registry key in example where there is no hive property
• Better handling of Sightings
• Add object data marking to Relationships when appropriate based on the source and target references
• Handle name and title properties of malware better

Testing changes

• Added testing for python 3.9
• Test for the ignore missing policy for STIX 2.1