Correctly reference `IPSet`

[dav3r]

🗣 Description

This PR fixes a bug by correctly referencing IPSet() instead of netaddr.IPSet().

💭 Motivation and context

When the netaddr import was modified in https://github.com/cisagov/cyhy-core/commit/a17267a2ccaf56f4e3ad51ccffbd2e8674e023f7, we overlooked changing this reference to IPSet. Without this change, you will get: NameError: global name 'netaddr' is not defined

🧪 Testing

I verified that this code change allowed me to run cyhy-import and have it run correctly and without the NameError: global name 'netaddr' is not defined that I was getting previously.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.