Fix module imports with Python3

Summary:
The broken module import was covered by the missleading warning
"WARNING Missing Graphviz dependency: diagrams will not be generated."

Test Plan: Installed and run kapidox_generate ../kunitconversion/

Reviewers: ochurlaud

Reviewed By: ochurlaud

Subscribers: kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D25632