The problem is that from VPP 18.07 onwards, the python api files are bunched under a top-level vpp_papi directory and this directory’s permissions, 644, do not allow non-root users access:

drw-r-r-.  3 root root   4096 Nov 23 09:52 vpp_papi

NOTE: Without the execute permissions, the directory is not accessible to non-root users and hence import of vpp_papi fails.

This causes devstack with VPP/networking-vpp to fail since devstack is run as non-root user.