The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The Python-Version or Python3-Version control field is not in one of the valid formats. It should be in one of the following:
all current current, >= X.Y >= X.Y >= A.B, << X.Y A.B, X.Y
(One or more specific versions may be listed with the last form.) A.B and X.Y should be Python versions.
Please refer to Debian Python Policy section 3.4 for details.