Bump tox from 4.18.1 to 4.20.0

Bumps [tox](https://github.com/tox-dev/tox) from 4.18.1 to 4.20.0.
- [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@4.18.1...4.20.0)

---
updated-dependencies:
- dependency-name: tox
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

setup.py

@@ -17,7 +17,7 @@
     packages=find_packages(),
     include_package_data=True,
     install_requires=requirements,
-    extras_require={"test": ["coverage==7.6.1", "pytest==8.3.3", "pytest-cov==5.0.0", "tox==4.18.1"]},
+    extras_require={"test": ["coverage==7.6.1", "pytest==8.3.3", "pytest-cov==5.0.0", "tox==4.20.0"]},
     entry_points="""
         [console_scripts]
         xml-validate=validator.__main__:cli