Use pytest instead of nosetests for tox python 2.7

if this works okay then will update others

tox.ini

@@ -11,8 +11,9 @@ commands =
 [testenv:py27]
 commands =
         pip install --upgrade pip
+        pip install --upgrade setuptools wheel
         pip install --upgrade --upgrade-strategy eager 'rattail[auth,bouncer,db,tests]' 'sqlalchemy-utils<0.36.7'
-        nosetests {posargs}
+        pytest {posargs}
 
 [testenv:coverage]
 basepython = python3