Envelope

- group syntax is no more reported as erroneous (ex: undisclosed-recipients:;)

- better invalid header loading (ex: invalid charset in the content type header)
- SMTP connection uses `ssl.create_default_context()` by default

- choose subject text placeholder while PGP encrypting
- fix sending e-mail without From if deliberately set to False (regression)

– Address properties are "" if empty (handy when comparing strings)
- replace invalid characters when loading (the way the e-mail clients behave)
- headers truly case-insensitive when loading
- `.from_()` and `.sender()` truly return an `Address` even if not set (and then the Address is empty)
- fix: the internal cache correctly regenerated
- message is internally held in bytes -> helpful error message when charset fails

- exposed Address
- Address casefold method and `user` and `host` properties
- loading ignores invalid headers (and prints out a warning)

- fix case-insensitive header loading
- fix non-latin chars in header loading