tox: use isolated builds

tox.ini

 [tox]
 [tox]
-skipsdist = True
+minversion = 3.3.0
+isolated_build = True
 envlist = py27, py35, py36, py37, py38
 envlist = py27, py35, py36, py37, py38
 
 
 [testenv]
 [testenv]
@@ -7,4 +8,4 @@ whitelist_externals = poetry
 skip_install = true
 skip_install = true
 commands =
 commands =
     poetry install -vvv
     poetry install -vvv
-    poetry run pytest tests/
+    poetry run pytest {posargs} tests/