NatLibFi/Annif: Annif 0.42

This release brings improvements to command line functionality and many robustness enhancements. The Docker images in the quay.io repository now have tags in line with release versions and the documentation in ReadTheDocs has both stable (latest release) and latest (current master) versions.There are also a lot of small improvements to the Docker image build pipeline. New features: #251/#306 CLI clear command for clearing project data Robustness improvements: #281/#299 Handle invalid lines in training data .tsv files #282/#300 Handling errors in projects.cfg file #325 Always use UTF-8 encoding when parsing TSV file document corpora #291/#296/#317 Pin down more specific versions of dependencies Bug fixes #303/#309/#313 Fix typos in documentation and bug in training from .key files with only subject labels #320 Turn AnnifException into an abstract base class and fix message prefix behavior Container pipeline and readthedocs build related fixes and enhancements 323, #312, #311, #310, #305, #304, #297/#298, #314, #319, #29