Avoid deprecated utcfromtimestamp in cache headers

[Mirochill]

Summary

• Import timezone from datetime.
• Build the cache If-modified-since datetime with datetime.fromtimestamp(..., tz=timezone.utc) instead of the deprecated datetime.utcfromtimestamp().

Closes #43

Validation

• rg -n utcfromtimestamp|fromtimestamp|timezone update-zonefile.py
• git diff --check
• Not run locally: update-zonefile.py / network-fetch workflow.