Following the guidelines to build Debian package from a Python files powered by Flassger.
When running a build getting an error:

ImportError: No module named swagger_spec_validator.util

Which mean, that test.py doesn't see swagger_spec_validator.
There seem to be no Swagger related pakcages for Debian at all. Should the swagger_spec_validator be included somewhere in debian/control file?
If swagger is the dependency of the software you package and it is not available in Debian,  You'll need to package it before.
If swagger is a dependency for test suite only, you may consider modifying or disabling the test.py by creating a patch in d/patches for example..
Python into Debian package: No module named error