Every raised exception leaks

PyErr_SetObject() takes its own reference to the value, so the one held by mariadb_throw_exception() and mariadb_exception_connection_gone() in mariadb_exception.c has to be dropped. It never is, so each raised exception is retained for the life of the process, along with its errmsg/msg/sqlstate/errno attributes.

Any application that hits errors in a loop grows without bound. Both functions also leak ErrorMsg, ErrorNo and SqlState on the return taken when the exception object can't be constructed.

2.0 already has both fixes.