AthConfigFlags: Fix for buggy hash function

This was factored out of !64979 (merged), where I implemented some unit tests for flag deletion. Turns out that our hash function cares a bit too much about the order of an internal dictionary.

Python dictionaries are insertion ordered (rather than random) as of python 3.7, but this means hash comparison in flags cares about the order in which they were created. I don't think this was what athHash was intended to check (but I could be wrong). If it isn't intended I suppose the reason it hasn't caused noticeable bugs is that we rarely change the order in which flags are inserted.