T252577: Tell about older tox versions no longer working (#125)

* T252577: Tell about older tox versions no longer working

* T252577: Do not mention specific tox version, simply use latest

* T252577: Mention the correct tox version, dont be vague

README.md

@@ -310,4 +310,9 @@ containing broken `External Method`s are recorded and played back. The same
 holds for `Z SQL Method`s which have `class_name` and `class_file` set to a no
 longer existing extension.
 
+Since `22.2.5` a more recent version ( `>= 3.26.0` ) of `tox` is required
+in order to build the test environment from `pyproject.toml` instead of
+`setup.py`. Do NOT get fooled by errors like `ERROR: No setup.py file found.`,
+just upgrade `tox` to latest version and retry.
+
 ## To Do / Roadmap