[3.7] bpo-33336: Legalize MOVE command (GH-6569)

[mcepl]

When running

    box = IMAP4_SSL(host)
    box.login(user, passw)
    msgs = somehowget_uids_of_messsages_to_move()
    box.uid('MOVE', msgs, 'target')

I get an error "Unknown IMAP4 UID command: MOVE". The problem is that imaplib contains a list Commands of the permitted IMAP commands, and MOVE is not included, even though it is perfectly legal (and quite widely supported) command according to RFC-6851.

@vstinner , @warsaw

https://bugs.python.org/issue33336

[vstinner]

I close this PR. I prefer to use miss-inlgton bot to generate backports PR to get "(cherry picked from commit XXX)" in the commit message.