The tag is present in Lintian version 2.109.6. That is the most recent version we know about.
The control fields
Python3-Version show the Python versions your package
supports, but the Python version listed here predates the current
"oldstable" distribution. There is no need to list it.
Please drop or update the named version.
When removing a version, please check
debian/tests/* for any use of
Python3-Version field, that program falls back to
all supported versions, which may not be what you want.
Please refer to Debian Python Policy section 3.4 for details.