RootUtils+PyUtils: Fix py38 memory corruption.

With python 3.8, the hacks used in pythonize_read_root_file lead to heap corruption. Rework to avoid this.

We're rather constrained since this has to work with both py2 and 3, so the way it's done here is a bit roundabout and ineffecient. Once we drop py2, this can be done much better by simply having read_root_file return a PyBytes object