The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
This package alternatively Build-Depends on the Python 2 or Python 3 version of the Sphinx documentation generator.
The 2.x series of Python is due for deprecation and will not be maintained by upstream past 2020 and will likely be dropped after the release of Debian "buster".
Please replace the alternative with a single build dependency on