maven: Fix last update datetime
We need to avoid using naive datetime as this fails during conversion.
Related to swh/infra/sysadm-environment#3746 (closed) Related to swh/meta$1269
Test Plan
tox
and docker run
complete with this
swh@60d6fea3afe1:/$ swh lister run -l maven url=https://repo1.maven.org/maven2/ index_url=http://swh-lister-maven-nginx/export.fld
INFO:swh.lister.maven.lister:Downloading text index from http://swh-lister-maven-nginx/export.fld.
INFO:swh.lister.maven.lister:Found 2 poms.
INFO:swh.lister.maven.lister:Fetching poms..
INFO:swh.lister.maven.lister:Fetching URL https://repo1.maven.org/maven2/al/aldi/sprova4j/0.1.0/sprova4j-0.1.0.pom with params {}
INFO:swh.lister.maven.lister:Fetching URL https://repo1.maven.org/maven2/al/aldi/sprova4j/0.1.1/sprova4j-0.1.1.pom with params {}
whereas it crashed without it (see paste in the diff description).
Migrated from D7139 (view on Phabricator)