Add linting for minimal supported Python version

(DIS-1491)

Author: pyrco

tox.ini

@@ -39,8 +39,10 @@ basepython = python3
 deps =
     flake8
     flake8-black
+    vermin
 commands =
     flake8 acquire tests setup.py
+    vermin -t=3.9- --no-tips --lint acquire tests setup.py
 
 [testenv:build]
 # Force the Python version here, so building will be done with the correct