Thanks. Will be tracking the issue.
If anyone else needs it, I am attaching an edit of the original build_manifest.py (https://github.com/pulp/pulp_rpm/blob/2.21-release/pulp_manifest/pulp_manifest/build_manifest.py) which should work on both Python2 (not tested) and Python3 which generates the PULP_MANIFEST file for provided directory.
Usage : build_manifest.py /var/lib/pulp/imports
Processing: build_manifest.py…
Processing: build_manifest…
build_manifest.tar (3.5 KB)