NatLibFi/Annif: Annif 0.47

This release changes the Python version requirement to 3.6+ and drops the usage of Pipenv in development installations. The TensorFlow library has been upgraded to version 2.2, which means that all features are now supported also under Python 3.8. The eval command is supplemented by introducing weighted subject average as a metric and the possibility to output metrics separately for each subject (thus allowing to explore e.g. how often a subject was suggested correctly); also some metrics are given more specified interpretation in the output. Other changes include the possibility to display notation codes (when available) in web UI as well as minor improvements, bug fixes, and maintenance tasks. New features: #392 Evaluate samples: specify interpretation of metrics (credit: @Veldhoen) #391/#393 Evaluation per-subject (credit: @Veldhoen) #390/#397 Show notations in web UI results list Improvements: #405/#403 Upgrade to TensorFlow 2.2, Python 3.6+, drop Pipenv #395/#396 Don't give suggestions for empty input #389/#401/#402 Improved error handling in maui backend #399 Miscellaneous minor improvements for readthedocs builds Maintenance: #400 Dockerfiles reorg and cleanup #407 Adding secrets needed by new Drone instance Bug fixes: #394 Fix click 7.1 compatibility in tests #398 Fix silently failing readthedocs build