Encode SQL statement as bytes in 0046_reviewrating database migration

An attempt to avoid a psycopg2 error about 'connection.encoding' not existing.

bookwyrm/migrations/0046_reviewrating.py

@@ -23,7 +23,7 @@ def convert_review_rating(app_registry, schema_editor):
             cursor,
             """
 INSERT INTO bookwyrm_reviewrating(review_ptr_id)
-VALUES %s""",
+VALUES %s""".encode(),
             values,
         )