Fix warning for joblib in scikit

[kinow]

Following the documentation and noticed the following warning:

/home/kinow/Development/python/workspace/Annif/venv/lib/python3.7/site-packages/sklearn/externals/joblib/__init__.py:15: DeprecationWarning: sklearn.externals.joblib is deprecated in 0.21 and will be removed in 0.23. Please import this functionality directly from joblib, which can be installed with: pip install joblib. If this warning is raised when loading pickled models, you may need to re-serialize those models with scikit-learn 0.21+.
  warnings.warn(msg, category=DeprecationWarning)

Not seen when running pytest.

[codecov]

Codecov Report

Merging #275 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #275   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          52       52           
  Lines        2624     2624           
=======================================
  Hits         2606     2606           
  Misses         18       18

Impacted Files	Coverage Δ
annif/project.py	100% <100%> (ø)	⬆️
annif/backend/pav.py	98.52% <100%> (ø)	⬆️

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[osma]

Thanks @kinow!