Use unittest discover in tox

tox.ini

@@ -2,8 +2,9 @@
 envlist = {py37,py38,py39,py310}-{sqlite,postgresql}
 
 [testenv]
+extras = test
 commands =
-    coverage run --include=.*/purchase_blanket_agreement/* setup.py test
+    coverage run --include=.*/purchase_blanket_agreement/* -m unittest discover -s tests
     coverage report --include=.*/purchase_blanket_agreement/* --omit=*/tests/*
 deps =
     coverage