Update and clean tox configuration for version 4

Related to swh/meta#4959 (closed)

mentioned in issue swh/meta#4959 (closed)

enabled an automatic merge when the pipeline for 1075e66c succeeds

Jenkins job DLDBASE/gitlab-builds #10 failed .
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Fixed by !454 (merged)

mentioned in merge request !455 (merged)

mentioned in merge request !454 (merged)

aborted the automatic merge because target branch was updated

added 3 commits

• 1075e66c...7c0a0d9f - 2 commits from branch swh/devel:master
• 741f874b - Update and clean tox configuration for version 4

Compare with previous version

enabled an automatic merge when the pipeline for 741f874b succeeds

Jenkins job DLDBASE/gitlab-builds #18 failed .
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@jenkins please retry the build

Jenkins job DLDBASE/gitlab-builds #19 failed .
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Jenkins job DLDBASE/gitlab-builds #20 failed .
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Jenkins job DLDBASE/gitlab-builds #21 succeeded .
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merged