RootUtils: python3 fixes
Add RootUtils::PyGetString to convert from a python string to a C++ string in a way that should work with py2, py3, and a module compiled with py2 running under py3.
Also clean up dependencies of the RootUtils libraries.
Add RootUtils::PyGetString to convert from a python string to a C++ string in a way that should work with py2, py3, and a module compiled with py2 running under py3.
Also clean up dependencies of the RootUtils libraries.