Helper script for next APE number

[pllim]

Would be nice if we have something like

https://github.com/astropy/astropy-tools/blob/main/next_pr_number.py

but for APEs, following:

• APE 1: Number is gone once reserved (naming chaos fix 2) #97

However, this script would be more complex given that it needs to find a way to reliably extract "reserved" numbers from all the proposals, past and present, given the inconsistency that have existed for many years.