fix tox 4 compatibility

[kmike]

I think we need o do it before the release, because CI is failing.

[codecov]

Codecov Report

Merging #209 (1b914c5) into master (b71f089) will not change coverage.
The diff coverage is n/a.

❗ Current head 1b914c5 differs from pull request most recent head 3eedf53. Consider uploading reports for the commit 3eedf53 to get more accurate results

@@           Coverage Diff           @@
##           master     #209   +/-   ##
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  Coverage   96.18%   96.18%           
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  Files           9        9           
  Lines         498      498           
  Branches       94       94           
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  Hits          479      479           
  Misses          9        9           
  Partials       10       10