chore(dependencies): update dependency tox to v4

Author: renovate[bot]

pyproject.toml

@@ -31,7 +31,7 @@ python = "^3.6"
 black = "^20.8b1"
 coverage = "^5.3.1"
 isort = "^5.8.0"
-tox = "^3.25.1"
+tox = "^4.15.0"
 tox-gh-actions = "^2.9.1"
 
 [build-system]