Allow passing parameters to pytest

An invocation like "tox -epy -- --pdb" will pass --pdb to pytest.

tox.ini

@@ -8,6 +8,6 @@ deps =
     coverage
     icalendar [test]
 commands =
-    coverage run --source=src/icalendar --omit=*/tests/* --module pytest src/icalendar
+    coverage run --source=src/icalendar --omit=*/tests/* --module pytest [] src/icalendar
     coverage report
     coverage html