This fixes the master build 
Build is green
Patch application report for D6666 (id=24231)
Rebasing onto 97553d8984...
Current branch diff-target is up to date.
Changes applied before test
commit 3ffea8f525cdd6bfc0df8ff526bf4622ca66787a Author: Antoine R. Dumont (@ardumont) <firstname.lastname@example.org> Date: Tue Nov 23 10:13:19 2021 +0100 lister: Fix type This fixes the master build   https://jenkins.softwareheritage.org/view/swh-draft/job/DLS/job/tests/1625/console
See https://jenkins.softwareheritage.org/job/DLS/job/tests-on-diff/413/ for more details.
mypy is right, last_modified is a date and iso8601.parse_date takes a date as string... 
Interesting, i did not knew that datetime.combine 
I've opened D6668 with that implementation so we have some clue.