python: define trade.AbstractSceneConverter before trade.SceneContents.

SceneContents.FOR() uses it as an argument. Wasn't caught by the doc
generator, and looks like it didn't break testing either, probably
because this is an overloaded function and when an overload gets picked,
the types are already defined. Or something. Not sure.

src/python/magnum/trade.cpp

@@ -3275,6 +3275,10 @@ void trade(py::module_& m) {
         .value("NONE", Trade::SceneConverterFlag{});
     corrade::enumOperators(sceneConverterFlags);
 
+    /* Has to be created before SceneContents as SceneContents.FOR() depends on
+       it */
+    py::class_<Trade::AbstractSceneConverter, PluginManager::PyPluginHolder<Trade::AbstractSceneConverter>, PluginManager::AbstractPlugin> abstractSceneConverter{m, "AbstractSceneConverter", "Interface for scene converter plugins"};
+
     py::enum_<Trade::SceneContent> sceneContents{m, "SceneContents", "Scene contents"};
     sceneContents
         .value("SCENES", Trade::SceneContent::Scenes)
@@ -3305,7 +3309,6 @@ void trade(py::module_& m) {
         });
     corrade::enumOperators(sceneContents);
 
-    py::class_<Trade::AbstractSceneConverter, PluginManager::PyPluginHolder<Trade::AbstractSceneConverter>, PluginManager::AbstractPlugin> abstractSceneConverter{m, "AbstractSceneConverter", "Interface for scene converter plugins"};
     abstractSceneConverter
         .def_property_readonly("features", [](Trade::AbstractSceneConverter& self) {
             return Trade::SceneConverterFeature(Containers::enumCastUnderlyingType(self.features()));