Update tox requirement in the all-dependencies group

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.

Updates `tox` 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@4.18.0...4.20.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements-build.txt

@@ -4,4 +4,4 @@ virtualenv~=20.26
 setuptools>=70.0.0 # not directly required, pinned by Snyk to avoid a vulnerability
 logilab-common~=2.0
 m2r~=0.3
-tox~=4.18
+tox~=4.20