Retry to get a connection when an OperationalError is raised

If there are no connection available in the pool, psycopg2 tries to create a
new connection but it may raise an OperationalError if the server close it.

Closes #13475
(grafted from cecbc1736f2d0fdd8a77c1dcdfe81e23cf99f451)

--HG--
branch : 7.0

trytond/trytond/backend/postgresql/database.py

@@ -259,7 +259,7 @@ class Database(DatabaseInterface):
         for count in range(retry, -1, -1):
             try:
                 conn = self._connpool.getconn()
-            except PoolError:
+            except (PoolError, DatabaseOperationalError):
                 if count and not self._connpool.closed:
                     logger.info('waiting a connection')
                     time.sleep(1)