Resolve DeprecationWarning by pytest: change warn to warning

[juhoinkinen]

This fixes one DeprecationWarning that pytest has been giving for a while:

tests/test_project.py::test_project_train_state_not_available
  /home/local/jmminkin/git/Annif/annif/project.py:175: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
    logger.warn('Could not get train state information.')

(Two other DeprecationWarnings remain, but they are from TensorFlow package.)

[codecov]

Codecov Report

Merging #449 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #449   +/-   ##
=======================================
  Coverage   99.43%   99.43%           
=======================================
  Files          65       65           
  Lines        4577     4577           
=======================================
  Hits         4551     4551           
  Misses         26       26           

Impacted Files	Coverage Δ
annif/project.py	99.34% <100.00%> (ø)

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication