Allow passing positional args to pytest via tox

tox.ini

@@ -6,8 +6,10 @@ deps=-r{toxinidir}/requirements/dev.pip
 whitelist_externals=
     /bin/sh
 commands=
+    # Allow a custom setup command to be specified via env vars,
+    # which is used in the `pre_twine` test below.
     /bin/sh -c "{env:PYPISERVER_SETUP_CMD:true}"
-    pytest []
+    pytest {posargs}
 sitepackages=False
 
 [pytest]