Update dependency tox to v4

Author: renovate[bot]

pyproject.toml

@@ -81,7 +81,7 @@ lockfile = "^0.12.2"
 recommonmark = "^0.7.1"
 psycopg2 = {version = "=2.9.6", python = "3.6.* || 3.7.* || 3.8.* || 3.9.*"}
 mysqlclient = {version = "=2.1.1", python = "^3.6"}
-tox = "^3.25"
+tox = "^4.0.0"
 mypy = {version = "^0.961", python = "^3.6"}
 django-stubs = {version = "^1.9", python = "^3.6"}