Log Message:
py-stix: updated to 1.2.0.10

Version 1.2.0.10
- Check add_reference methods to prevent NoneType has no attribute 'append'
- Changes to STIXPackage to prevent the empty <stix:TTPs/> tag from \ 
appearing in serialization

Version 1.2.0.9
- TTPs would fail to serialize XML Kill_Chains if no TTP was set
- Added Python 3.8 to test harness

Version 1.2.0.8
- Add xnl:Type to the PersonName element (CIQ)
- Update the allowable values for PersonName and OrganisationName
- Update tests per recent CybOX release

Version 1.2.0.7
- Update package requirements