Add is_final to disallow inheritance from Python

- Not currently supported on PyPy

tests/test_class.py

@@ -279,3 +279,21 @@ def test_aligned():
     if hasattr(m, "Aligned"):
         p = m.Aligned().ptr()
         assert p % 1024 == 0
+
+
+# https://bitbucket.org/pypy/pypy/issues/2742
+@pytest.unsupported_on_pypy
+def test_final():
+    with pytest.raises(TypeError) as exc_info:
+        class PyFinalChild(m.IsFinal):
+            pass
+    assert str(exc_info.value).endswith("is not an acceptable base type")
+
+
+# https://bitbucket.org/pypy/pypy/issues/2742
+@pytest.unsupported_on_pypy
+def test_non_final_final():
+    with pytest.raises(TypeError) as exc_info:
+        class PyNonFinalFinalChild(m.IsNonFinalFinal):
+            pass
+    assert str(exc_info.value).endswith("is not an acceptable base type")