chore(deps): update dependency tox to v4.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
tox (changelog)	==4.20.0 -> ==4.21.0

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Release Notes

tox-dev/tox (tox)

### [`v4.21.0`](https://redirect.github.com/tox-dev/tox/compare/4.20.0...4.21.0) [Compare Source](https://redirect.github.com/tox-dev/tox/compare/4.20.0...4.21.0)

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Configuration

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🚦 Automerge: Enabled.

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