Avoid PytestUnknownMarkWarning due to "slow" marker

[juhoinkinen]

Register "slow" test marker as described in pytest documentation to avoid the warning:

tests/test_openapi.py:22
  /home/local/jmminkin/git/Annif/tests/test_openapi.py:22: PytestUnknownMarkWarning: Unknown pytest.mark.slow - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @pytest.mark.slow

The "slow" marker was added in #685.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (40cc2fd) 99.67% compared to head (468e263) 99.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #728   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files          89       89           
  Lines        6397     6397           
=======================================
  Hits         6376     6376           
  Misses         21       21           

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