tests: Pass additional tox args to pytest

This allows to run specific tests e.g. via `tox -e py3
tests/test_servefile.py::test_serve_relative_directory`.

tox.ini

@@ -5,4 +5,4 @@ envlist = py27,py36
 deps =
         pytest
         requests
-commands = pytest --tb=short
+commands = pytest --tb=short {posargs}