Pythonizations that are common to TTree and its subclasses were injected
by checking if the given class `InheritsFrom` TTree. This operation is
costly and, since it is executed for every new class that is referenced
from the user application, it triggers the loading of libTree even if
the user Python script does not use any class from it.

The proposed solution uses an explicit list of classes to be pythonized.
While being less flexible (subclasses are not automatically picked and
pythonized) it does not suffer from the aforementioned issues.

Note that, if in the future PyROOT allows users to define their own
pythonizations, user classes that subclass TTree will still be able to
inject any desired behaviour, including the pythonizations of TTree.