Co-authored-by: Jan Knedlik <j.knedlik@gsi.de>

Co-authored-by: Jan Knedlik <j.knedlik@gsi.de>

Cppyy allows to add custom pythonisations for classes, but it does
not yet provide an API for adding custom type converters.

Until that API exists, we need to patch Cppyy with a converter
Python string -> TString, so that we do not have to create a
TString in Python when calling a C++ method that expects it.