Use the referencing library

[matthewrmshin]

Description

This is to avoid the DeprecationWarning about
retrieving remote references being a security vulnerability.

(We have only used this feature for referencing items in the local file system, so the software should not have been compromised. Nevertheless it is best to modify the code so we don't get this warning.)

Impact

Expect no more deprecation warning.

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have run the unit tests before creating the PR

[matthewrmshin]

Sorry the fix is so complicated, but I don't think I can do any better.

This was the warning:

DeprecationWarning: Automatically retrieving remote references can be a security vulnerability and is discouraged by the JSON Schema specifications. Relying on this behavior is deprecated and will shortly become an error. If you are sure you want to remotely retrieve your reference and that it is safe to do so, you can find instructions for doing so via referencing.Registry in the referencing documentation (https://referencing.readthedocs.org).

[steoxley]

I've taken a read through this, and discussed it with the @matthewrmshin, I'm happy that this is probably the best we can do at this point if we don't want users to see security warnings. When we no longer support python 3.7 the code can be refactored to look less cumbersome.