NatLibFi/Skosify: Skosify 2.3.0

This is a minor version that makes some operations (prefLabel selection, logging) more deterministic, helping to ensure that successive runs of Skosify on the same input produce the same result every time. A new facility for performing SPARQL Update operations just before serializing the output has been added.

On the more technical side, Skosify now works with rdflib 6 as well, Python 2.7 support has been dropped, support for Python 3.9 introduced, and there are some changes to the unit tests and CI environment.

• #79/#80 Deterministic skosify log
• #85 add post_update_query option
• #87 Fix compatibility with rdflib 6.0.0
• #88 Drop Python 2 compatibility, add Python 3.9 support
• #89 Switch from Travis CI to GitHub Actions
• #90 Upgrade pytest and switch to flake8
• #91 Make prefLabel policies deterministic by using label string sort order to break ties