Make importlib_metadata regular dependency

[juhoinkinen]

When some DeprecationWarnings of unit tests were resolved in #575, I put importlib_metadata installation under dev dependencies, but it should be a regular dependency.

[sonarcloud]

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0.0% Duplication

[osma]

Can you clarify why it should be a regular dependency?

[codecov]

Codecov Report

Merging #580 (b2d3ad2) into master (3629139) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #580   +/-   ##
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  Coverage   99.47%   99.47%           
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  Files          84       84           
  Lines        5568     5568           
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  Hits         5539     5539           
  Misses         29       29           

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[juhoinkinen]

Uups, now when I checked why it should be a regular dependency, I see no reason.

I thought that importlib_metadata was used in the annif --version CLI option, but it is not; it is used only in testing it, so it should remain as a dev dependency.

I'll close this, thanks for questioning :)