svn.loader: Fix revision parents field to a tuple
Changing only the converters because that loader is currently mostly not typed... (so modifying and adding type along sounds like taking too much time than needed right now).
Also manipulating list within the loader sounds simpler than tuple. So taking the lazy path here.
Related to swh-model!113 (closed)
Test Plan
tox
Migrated from D3219 (view on Phabricator)