parse_datetime fails to parse RFC 5322 ("Internet Message Format", ye olde email) dates

Example: "Thu, 11 Jan 2024 21:17:19 +0100"

This format is produced (`date --rfc-email`) and understood (`date --date=...`) by GNU `date`.

See https://datatracker.ietf.org/doc/html/rfc5322#section-3.3 for the spec.