The programming language really does not matter when talking to a JSON-RPC endpoint so use whatever knowledge you already have invested in.
I have not done any python in the last 10 years or so but it has become increasingly popular lately so I assume that would do what you need.
I cannot vouch for it but there is at least this jsonrpcclient
library on the PyPi repository that might help.
It really also depends on your needs. You mentionned they were sending you an XML file that you were using already. You probably have some script reading it I assume for integration.
You could probably just modify that script to use the JSON-RPC api instead assuming they also have and endpoint that can get you a list of all products.
Are you trying to integrate it with your own inventory system? An e-commerce? etc
For testing the API, you can use something like json-rpc-2.0-shell
to query the API directly from a simple command shell.
Feel free to concact me directly as well.