Update tox requirement from ==4.15.* to ==4.21.*

[dependabot[bot]]

Updates the requirements on tox to permit the latest version.

Changelog

Sourced from tox's changelog.

v4.21.0 (2024-09-30)

Features - 4.21.0

- Native TOML configuration support - by :user:`gaborbernat`. (:issue:`999`)
Improved Documentation - 4.21.0

• Update Loader docs - by :user:ziima (:issue:3352)

v4.20.0 (2024-09-18)

Features - 4.20.0

- Separate the list dependencies functionality to a separate abstract class allowing code reuse in plugins (such as
  ``tox-uv``) - by :gaborbernat`. (:issue:`3347`)
v4.19.0 (2024-09-17)
Features - 4.19.0

• Support pypy-<major>.<minor> environment names for PyPy environments - by :user:gaborbernat. (:issue:3346)

v4.18.1 (2024-09-07)

Bugfixes - 4.18.1

- Fix and test the string spec for the ``sys.executable`` interpreter (introduced in :pull:`3325`)
  - by :user:`hroncok` (:issue:`3327`)
Improved Documentation - 4.18.1

• Changes the tox_env_teardown docstring to explain the hook is called after a tox env was teared down. (:issue:3305)

v4.18.0 (2024-08-13)

Features - 4.18.0

- Suppress spinner in parallel runs in CI - by :user:`ziima`. (:issue:`3318`)
Bugfixes - 4.18.0

• Boost temporary directories cleanup in tests - by :user:ziima. (:issue:3278)
• Fix absolute base python paths conflicting - by :user:gaborbernat. (:issue:3325)

... (truncated)

Commits

• 46bef9e release 4.21.0
• cea105b True TOML config support (#3353)
• a04b039 [pre-commit.ci] pre-commit autoupdate (#3385)
• f5eba31 Update Loader docs (#3352)
• 329db63 [pre-commit.ci] pre-commit autoupdate (#3350)
• f4b3bd2 Bump pypa/gh-action-pypi-publish from 1.10.1 to 1.10.2 (#3349)
• a04cc3a release 4.20.0
• 4d8f5fd Separate list dependencies to a separate installer class (#3347)
• 0cb816c YML to YAML
• 2f83624 release 4.19.0
• Additional commits viewable in compare view

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