[NEW PORT] mail/getmail6: POP3/IMAP4 mail retriever (Python 3 fork)

POP3/IMAP4 mail retriever with Maildir and command delivery (python3 fork).

Summary of features:

- Retrieve mail from an unlimited number of POP3/IMAP4 mailboxes and servers.
- Support for POP3-over-SSL and IMAP-over-SSL, as well as SDPS
  WARNING: even with SSL, "no certificate or key validation is done." so
  that getmail does not detect or protect from man-in-the-middle attacks.
- Support for multidrop or domain mailboxes.
- Safe and reliable delivery to qmail-style Maildirs, as well as program
  (pipe) delivery for use with arbitrary external MDAs. Includes an MDA
  for mbox files that supports mboxrd format and fcntl-type flock locking.
- Does not destroy information by rewriting mail headers.
- Does not cause mail loops by doing SMTP injection, and therefore does
  not require that you run an MTA (like qmail or sendmail) on your host.
- Can remember which mail it has already retrieved, and can be set to
  only download new messages.
- Written in Python, and therefore easy to extend or customize.
- Simple to install, configure, and use.

WWW: https://github.com/getmail6/getmail6/
PR: 248633
Submitted by: Michael Bueker <m.bueker@berlin.de> (maintainer)
Differential_Revision: D26057