Fix tox.ini (backport #9276)

[mergify]

This is an automatic backport of pull request #9276 done by Mergify.
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	both modified:   tox.ini

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[t-imamichi]

#9272 is necessary for stable too. The following error occurs.

DeprecationWarning: Widget._active_widgets is deprecated.

[t-imamichi]

Shall I merge #9279 to this PR? We need both to pass CI.