Update tox configuration file for tox 4.0.2 (backport #16)

[mergify[bot]]

This is an automatic backport of pull request #16 done by Mergify.

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More conditions and actions can be found in the [documentation](https://docs.mergify.com/). You can also trigger Mergify actions by commenting on this pull request: - `@Mergifyio refresh` will re-evaluate the rules - `@Mergifyio rebase` will rebase this PR on its base branch - `@Mergifyio update` will merge the base branch into this PR - `@Mergifyio backport ` will backport this PR on `` branch Additionally, on Mergify [dashboard](https://dashboard.mergify.com/) you can: - look at your merge queues - generate the Mergify configuration with the config editor. Finally, you can contact us on https://mergify.com

[coveralls]

Pull Request Test Coverage Report for Build 3657925759

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 69.697%

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Totals
Change from base Build 3633132359:	0.0%
Covered Lines:	69
Relevant Lines:	99

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💛 - Coveralls