Ietfdata

- Be more robust downloading mail archives

- Fix bluesheet document URLs

- Fix potential IMAP timeout
- Add `on_agenda` field to `Session` class, bump cache version

- Drop required Python version down to 3.8

- Allow empty event type when fetching document events

- Update cache support
- Expand parameters for `person_aliases()`
- Expand parameters for `people()`
- Add `person_from_name_email()` to DatatrackerExt
- Rename `MailingList`-related types and methods in Datatracker to
`EmailList` to make it easier to use DataTracker and MailArchive
classes together
- Add methods and types relating to countries and continents
- Update mail archive support