ZMySQLDA: reconnect on mariadb's Connection was killed

After server restarts, mariadb uses an error 1927 which is mariadb
specific, with no corresponding constant in CR module when server
restart. This is the case at least with mariadb 10.3.38.

This fix the following problem:
 - start zope and use mariadb connection
 - restart mariadb
 - use mariadb connection again, see an error:
    Error Type: OperationalError
    Error Value: (1927, 'Connection was killed')

This change to treat 1927 as an error for which we can reconnect and
retry the query. Some other connectors do it, for example sqlalchemy in
https://github.com/sqlalchemy/sqlalchemy/issues/5493

product/ZMySQLDA/db.py

@@ -116,7 +116,8 @@ from ZODB.POSException import ConflictError
 hosed_connection = (
     CR.SERVER_GONE_ERROR,
     CR.SERVER_LOST,
-    CR.COMMANDS_OUT_OF_SYNC
+    CR.COMMANDS_OUT_OF_SYNC,
+    1927, # ER_CONNECTION_KILLED "Connection was killed" in MariaDB
     )
 
 query_syntax_error = (