[PyROOT] Compensate for the absence of instantiated templates

After the changes proposed by PR#3858, TClingMethodInfo does not
iterate over instantiated templates anymore. This requires
changes in PyROOT, since GetListOfMethods will not return those
instantiations.

Instead of relying on the instantiations for the lookup of a
given function/method to succeed, the list of function templates
needs to be checked too and create a TemplateProxy if necessary.