Fix license statements on db_utils.py
When the code was migrated from swh.storage, some other ancillary functions were merged into this file. So bits of it are under the LGPLv3 (the code pulled from psycopg2) and bits are under the GPLv3 (the code we wrote).
Test Plan
only comment shuffling
Migrated from D3987 (view on Phabricator)