Thanks, it works this way :-)
To complete it here for the testcase above in case of still using
12.1:
In order to be able to drop the oldest and last range partition, we need to execute the interval command once again. This converts all INTERVAL partitions to RANGE Partitions and so we have again more than 1 RANGE partition.
ALTER TABLE T SET INTERVAL (2);
Disadvantage of this is only, that in case of gaps between interval partitions, they might get converted to bigger ranges than they were intended before. But in most cases, it won't be a problem.