make `python manage.py pgpartition` smarter

[j4tmr]

Currently, when adding new range partition will not acquire the old partition and automatically distribute the time evenly. For example, if i add 10 partitions ahead with 3 months seperated, next time i have to check the start time using PostgresTimePartitioningStrategy or it will occur a overlap partition error. Because range 2022-6-1 to 2022-9-1 and range 2022-5-1 to 2022-8-1 are both accpetable for the 3 months range partition.

[Photonios]

Can you please show your partitioning config? This sounds like a misconfiguration.