eliminate one form of PythonObject::Reset()

Summary:
I'd like to eliminate all forms of Reset() and all public constructors
on these objects, so the only way to make them is with Take<> and Retain<>
and the only way to copy or move them is with actual c++ copy, move, or
assignment.

This is a simple place to start.

Reviewers: JDevlieghere, clayborg, labath, jingham

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69080

llvm-svn: 375182