Python: Tweak handling of python project files

Improves the handling of the preferred python project
file (*.pyproject) and makes use of the json support.

Change-Id: I24d6e2c1d10899efacec0fc9b03660bb8f25dfe7
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/python/pythonplugin.cpp

@@ -59,6 +59,7 @@ void PythonPlugin::initialize()
     d = new PythonPluginPrivate;
 
     ProjectManager::registerProjectType<PythonProject>(PythonMimeType);
+    ProjectManager::registerProjectType<PythonProject>(PythonMimeTypeLegacy);
     JsonWizardFactory::registerPageFactory(new PythonWizardPageFactory);
 }