There's a limit in pycharm on a file length for indexing. By default it's 2.56 and recently hou.py just exceeds that limit
So to fix that go to Help -> Edit Custom Properties (it asks to create one if it doesn't exist, just hit Create)Image Not Found
Then put this line to idea.properties:idea.max.intellisense.filesize=3000Image Not Found
3000 is just an example you can make it higher to prevent problems in future
Then go to File -> Invalidates Cache / Restart. That should fix the issue with indexing (so path to hou.py has to be configured correctly and you need only $HFS/houdini/python2.7libs btw)
This is the one that worked for me in combination with setting Hython to the interpreter. Not sure why it says Hython 3.11 when i only have Python 3.9 and chose Hython39.exe but i digress. Got it working with the python3.9libs too. Also looks like 3000 was not enough so i put it to 30000 cos i'm chaotic in nature.