Update tox requirement from <4.0.0 to <5.0.0 in /requirements

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@0.5...4.15.1)

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

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

requirements/tests.txt

@@ -1,4 +1,4 @@
 -r dev.txt
 coverage==6.3.2
 requests-mock==1.9.3
-tox<4.0.0
+tox<5.0.0