[libcalamares] Untangle Python includes

- Use BoostPython.h to manage overall includes
 - Remove local home-grown variations

src/libcalamares/PythonHelper.cpp

@@ -25,14 +25,6 @@
 #include <QDir>
 #include <QFileInfo>
 
-#undef slots
-#include "utils/boost-warnings.h"
-#include <boost/python.hpp>
-
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif
-
 namespace bp = boost::python;
 
 namespace CalamaresPython