[libcalamares] The script namespace is actually a dict

Use dict methods, in particular d.get(k, v), to retrieve
the pretty_name() function (or None if it isn't there).
Using getattr() on a dict will not return values in the
dict.

src/libcalamares/PythonHelper.cpp

@@ -233,7 +233,7 @@ Helper::~Helper()
 {}
 
 
-boost::python::object
+boost::python::dict
 Helper::createCleanNamespace()
 {
     // To make sure we run each script with a clean namespace, we only fetch the