Investigate TLD cache warning

[robertknight]

The following warning was observed from viahtml in production, when it runs in Docker:

eb (stderr)         | 2022-07-22 05:00:18,274: [WARNING]: unable to cache TLDs in file /usr/local/lib/python3.8/site-packages/tldextract/.tld_set. This will refresh the Public Suffix List over HTTP every app startup. Construct your `TLDExtract` with a writable `cache_file` or set `cache_file=False` to silence this warning. [Errno 13] Permission denied: '/usr/local/lib/python3.8/site-packages/tldextract/.tld_set'

I'm unclear whether this is an issue in Via or a library it uses (thoughts @jon-betts?). Either way it would be good to either resolve the issue (so TLDs are cached) or decide that we don't need caching and suppress the warning.