Does python-bitcoinlib and the Bitcoin Core functional test framework share code? Were changes ever pushed down/upstream? Does python-bitcoinlib and the Bitcoin Core functional test framework share code? Were changes ever pushed down/upstream?
script.py of the Bitcoin Core functional test framework contains a comment that it was initially modified from python-bitcoinlib.
There doesn't appear to have been any code sharing beyond that and certainly not any later changes pushed up/downstream.
I asked Kanzure (previous maintainer of python-bitcoinlib) on IRC about it and he said:

python-bitcoinlib is forked from a thing that was itself incorporated into bitcoin core's functional test framework or the thing it was forked from was from code in the bitcoin core repository, i forget the lineage exactly but python-bitcoinlib came later