Fixes issue 132 (Public Suffix List (PSL) Updates Too Often)

🗣 Description

Added code to set the PSL file's last modified time to reflect time of last download.

💭 Motivation and context

PSL should only be downloaded once per day. The current code attempts to enforce that by only downloading a new PSL, when the local copy is older than 24 hours:

psl_age = datetime.now() - datetime.fromtimestamp(
    stat(PublicSuffixListFilename).st_mtime
)
if psl_age > timedelta(hours=24):
    updatePSL(PublicSuffixListFilename)

stat.st_mtime checks the last modified date of the local PSL, however, the last modified date will often be 2-4 days ago even if the PSL was downloaded minutes before.

Resolves #132.

🧪 Testing

I ran Trustymail against an array of domains. The first domain was tested and then the code waited for a local PSL to be created before testing the remaining domains. I logged all STDOUT to see if the "PSL Updated" message appeared more than once.

✅ 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