use keepalive-socket for cross-platform support

[tvogel]

On Linux, I received a

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/til/git/MailboxCleanup/src/mailbox_cli.py", line 133, in <module>
    main()
  File "/home/til/git/MailboxCleanup/src/mailbox_cli.py", line 102, in main
    imap.login()
  File "/home/til/git/MailboxCleanup/src/mailbox_imap.py", line 84, in login
    self.imap.sock.setsockopt(socket.IPPROTO_TCP, TCP_KEEPALIVE, 3)
OSError: [Errno 22] Invalid argument

So, I looked for a cross-platform solution, found https://stackoverflow.com/a/14855726/119725 and https://stackoverflow.com/a/76668530/119725 and decided to use the latter.

I hope, you find it useful. Thanks for the great project!

PS. Sorry for the PR noise - I used main as the source branch before and that is not good...