Tox: properly install the test extra.

Otherwise you get crazy errors.

tox.ini

@@ -7,7 +7,8 @@ deps =
     pytest
     coverage
     py{27,36}: hypothesis>=3.0
-    .[test]
+extras =
+    test
 commands =
     coverage run --source=src/icalendar --omit=*/tests/* --module pytest []
     py{27,36}: coverage run --append --source=src/icalendar --omit=*/tests/* --module pytest [] src/icalendar/tests/hypothesis/