Update tox requirement from ==4.13.* to ==4.15.* (#180)

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@4.13.0...4.15.0)

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

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

Author: dependabot[bot]

requirements-test.txt

@@ -1,4 +1,4 @@
 coverage == 7.4.*
 mypy == 1.10.*
 pre-commit == 3.5.*
-tox == 4.13.*
+tox == 4.15.*