eliminate virtual methods from PythonDataObjects

Summary:
This patch eliminates a bunch of boilerplate from
PythonDataObjects, as well as the use of virtual methods.
In my opinion it also makes the Reset logic a lot more
clear and easy to follow.   The price is yet another
template.   I think it's worth it.

Reviewers: JDevlieghere, jasonmolenda, labath, zturner

Reviewed By: JDevlieghere, labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68918

llvm-svn: 374916