db_utils: Fix typing of db/conninfo objects

swh_set_db_module() takes a db_or_conninfo object, and passes it to execute_sqlfiles, which only expects a conninfo.

This is detected by mypy when using a recent version of types-psycopg2.

@bchauvet This takes care of the other issue you had

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Migrated from D7282 (view on Phabricator)