I suspect this isn’t possible, unfortunately. Looking at test_hash.py the HashRandomizationTests class and its descendants were added in the commit that introduced this behavior. They test the hashing behavior by modifying the environment and starting a new process with PYTHONHASHSEED explicitly set. You could try to copy that pattern, perhaps.
I also just noticed you said “Every time I run my script, dict contents are iterated in a different order.” – I assume you’re aware of collections.OrderedDict, right? That’s the normal way to get reliable hash iteration.
If you’re willing to set the value in your shell environment, you could also just wrap your python call in a bash script, e.g.
#! /bin/bash
export PYTHONHASHSEED=0
# call your python program here
That avoids needing to manipulate your whole environment, as long as you’re ok with a wrapper script.
Or even just pass the value on the command line:
$ PYTHONHASHSEED=0 python YOURSCRIPT.py