Holy smokes!
The problem is that, even though the compiled pathlib library is in the python 3.8.5 zip file, there’s also a pathlib.py file in the pyRevit-master\site-package folder that takes precedence!
I tried to remove the file, restart pyrevit and langchain imports fine!
This is definitely a bug, and brings up the broader question of how are we going to handle both python version 2 and 3 packages!