Python: Create a default BuildSystem for Python targets

This makes sure the project can be parsed even if we do not have a valid
build configuration for that target.

Amends 09e94ae4ac

Change-Id: I92214474f581af228bd5c2aaf2f3e4b620ffc9d5
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonproject.cpp

@@ -23,6 +23,8 @@ PythonProject::PythonProject(const FilePath &fileName)
     setId(PythonProjectId);
     setProjectLanguages(Context(ProjectExplorer::Constants::PYTHON_LANGUAGE_ID));
     setDisplayName(fileName.completeBaseName());
+
+    setBuildSystemCreator([](Target *t) { return new PythonBuildSystem(t); });
 }
 
 Project::RestoreResult PythonProject::fromMap(const Store &map, QString *errorMessage)