Reject control characters in IMAP commands

[sethmlarson]

Linked PRs

• gh-143921: Reject control characters in IMAP commands #143922

[bitdancer]

If there is someone familiar enough with IMAP to review this, please do.

Otherwise I need a bit more detail, since I'm not that familiar with IMAP, and both the RFC and the code are quite complex. Can you cover how the RFC makes non-printables (other than space) illegal in 'commands', and how that relates to the imaplib implementation of _command? I scanned the RFC and code for a bit, and while my gut says your PR is correct, it isn't obvious by any means.