Switch helper package for the Public Suffix List

🗣 Description

This pull request switches Public Suffix List helper packages from publicsuffix to publicsuffixlist. Any necessary supporting changes are also made.

💭 Motivation and context

The publicsuffix package is deprecated and has not been maintained for years. It finally reached a breaking point during this past weekend's BOD 18-01 scanning run where the server hosting the maintained public suffix list no longer returned the charset header that publicsuffix expects. The publicsuffixlist package is the currently maintained one of the two alternatives mentioned by publicsuffix and it has a compatibility layer for existing code that uses publicsuffix.

🔔 See also

🧪 Testing

Automated tests pass. I confirmed that I was able to perform a local scan using this branch.

✅ 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.

✅ Pre-merge checklist

[x] Finalize version.

✅ Post-merge checklist

[x] Create a release.

cisagov / trustymail