chore(deps-dev): update tox requirement from ~=4.20 to ~=4.21 (appium#1037)

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/main/docs/changelog.rst)
- [Commits](tox-dev/tox@4.20.0...4.21.0)

---
updated-dependencies:
- dependency-name: tox
  dependency-type: direct:development
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Pipfile

@@ -15,7 +15,7 @@ pylint-quotes = "~=0.2.3"
 pytest = "~=8.3"
 pytest-cov = "~=4.1"
 python-dateutil = "~=2.9"
-tox = "~=4.20"
+tox = "~=4.21"
 types-python-dateutil = "~=2.9"
 
 [packages]