Add subclassing testcase which previously caused a segfault

Add a testcase from #407. Both test cases don't segfault after the
change to the #[pyclass] system in #683.

Closes #407

examples/rustapi_module/tests/test_subclassing.py

@@ -9,6 +9,7 @@ class SomeSubClass(Subclassable):
     pass
 
 
+def test_subclassing():
     if not PYPY:
         a = SomeSubClass()
         _b = str(a) + repr(a)