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

[dependabot]

Updates the requirements on tox to permit the latest version.

Release notes

Sourced from tox's releases.

4.21.2

What's Changed

• Update sdist rules to include tox.toml (#3389) by @​mgorny in tox-dev/tox#3390

Full Changelog: tox-dev/tox@4.21.1...4.21.2

Changelog

Sourced from tox's changelog.

v4.21.2 (2024-10-03)

Bugfixes - 4.21.2

- Include ``tox.toml`` in sdist archives to fix test failures resulting from its lack.
  - by :user:`mgorny` (:issue:`3389`)
v4.21.1 (2024-10-02)
Bugfixes - 4.21.1

• Fix error when using requires within a TOML configuration file - by :user:gaborbernat. (:issue:3386)

• Fix error when using deps within a TOML configuration file - by :user:gaborbernat. (:issue:3387)

• Multiple fixes for the TOML configuration by :user:gaborbernat.:

  • Do not fail when there is an empty command within commands.
  • Allow references for set_env by accepting list of dictionaries for it.
  • Do not try to be smart about reference unrolling, instead allow the user to control it via the extend flag, available both for posargs and ref replacements.
  • The ref replacements raw key has been renamed to of. (:issue:3388)

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)

... (truncated)

Commits

• c953f4f release 4.21.2
• 3ab1d3e Update sdist rules to include tox.toml (#3389) (#3390)
• 172a952 Fix user guide typo
• 434ac18 release 4.21.1
• 34d3adc Fix TOML configuration errors (#3388)
• 719b346 Update installation.rst
• 46bef9e release 4.21.0
• cea105b True TOML config support (#3353)
• a04b039 [pre-commit.ci] pre-commit autoupdate (#3385)
• f5eba31 Update Loader docs (#3352)
• Additional commits viewable in compare view

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[dependabot]

Superseded by #112.