That fixes the build [1] to unstuck @anlambert.
I'm not entirely satisfied though but i don't see how to fix it otherwise. I
don't really get what's pulling this (I see nothing explicitely requiring
something from the core.db...).
D4300 (included in swh.core 0.6.1) does introduce the new postgresql_fact
fixture factory (to make pg db tests using the built fixture ouf of that
factory faster). Which pulls the pytest_postgresql as runtime dependency...
[1] https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/1743/console