PyUtils: Fix the diff-root dictionary problem
In some CI tests, we've been seeing failures in diff-root due to dictionary problems when retrieving values from a TTree. Earlier we tried to work around this by first fetching the TClass for the involved class, but that didn't work. However, if we go all the way to making a Python class, then things seem to work.
Also delete the extra diagnostic code that was added earlier.
See ATEAM-1000.