Pass posargs to pytest in main testenv

tox.ini

@@ -15,7 +15,7 @@ install_command =
 commands_pre =
     find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -path '*/__pycache__/*' -name '*.py[c|o]' -delete
 commands =
-    pytest
+    pytest {posargs}
     py36,py37: black --check .
     isort --recursive --check-only --settings-path setup.cfg .
     flake8 .