The tag is present in Lintian version 2.108.13. That is the most recent version we know about.
The package specifies a dependency on Python 2.x which is due for deprecation and will not be maintained upstream past 2020 and will likely be dropped after the release of Debian "buster".
You should not make any changes to your package based on this presence of this tag.
However, please override this tag with a suitably-commented override if it is known that this package will not be migrated to Python 3.x for one reason or another. This is so that developers may ignore the package when looking for software that needs to be ported.
Please refer to https://wiki.debian.org/Python/Python3Port for details.
This tag is experimental.