Run tests for all installed versions of Python (#166).

tox.ini

@@ -1,5 +1,6 @@
 [tox]
-envlist = py3
+envlist = py35,py36,py37
+skip_missing_interpreters=true
 skipsdist = True
 minversion = 3.10.0
 
@@ -12,8 +13,8 @@ install_command =
     python -m pip install --no-use-pep517 {opts} {packages}
 commands =
     pytest
-    sh scripts/black
-    sh scripts/isort
+    py36,py37: black --check .
+    isort --recursive --check-only --settings-path setup.cfg .
     flake8 .
 
 [testenv:black]