plugin XEP-0346: fix deprecated import

sat/plugins/plugin_xep_0346.py

@@ -16,7 +16,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from collections import Iterable
+from collections.abc import Iterable
 import itertools
 from typing import Optional
 from zope.interface import implementer