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.

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[codecov[bot]]

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

```diff @@ 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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