Use PythonDataObjects in swig helper functions.

Relying on manual Python C API calls is error prone, especially
when trying to maintain compatibility with Python 2 and Python 3.

This patch additionally fixes what appears to be a potentially
serious memory leak, in that were were incref'ing two values
returned from the session dictionary but never decref'ing them.
There was a comment indicating that it was intentional, but the
reasoning was, I believe, faulty and it resulted in a legitimate
memory leak.

Switching everything to PythonObject based classes solves both
the compatibility issues as well as the resource leak issues.

llvm-svn: 252536