Bug#956190: dh-python's autopkg tests fail with python3.8 as the only supported python3 version

[Olivier Tilloy]

This is fixed with
https://salsa.debian.org/python-team/tools/dh-python/-/commit/e289c25c861ae65ae9e7b52ebc09cf1f56061fa7,
but that version hasn't been released yet.