[libcalamares] Reduce warnings with Clang 9 (Boost::Python)

- The boost headers fall foul of tons of Clang warnings, so introduce
   a mechanism like moc-headers for turning off most of those warnings.

src/libcalamares/PythonHelper.cpp

@@ -26,8 +26,13 @@
 #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