message: db_engine_specs: use correct sqlite week time grain
  We want the week of year %W and not the day of week %w
  when using week as time grain.
  Reference:
diff: @@ -424,7 +424,7 @@ class SqliteEngineSpec(BaseEngineSpec):
  'PT1H'),
  Grain('day', _('day'), 'DATE({col})', 'P1D'),
  Grain('week', _('week'),
  - "DATE({col}, -strftime('%w', {col}) || ' days')",
  + "DATE({col}, -strftime('%W', {col}) || ' days')",
  'P1W'),
  Grain('month', _('month'),
  "DATE({col}, -strftime('%d', {col}) || ' days', '+1 day')",
  