docs: imaplib: use the reST :class: role, escaped

docs: imaplib: use the reST :class: role, escaped

This is a different approach to f385e441df15, which was reverted for
creating dangling link references.

By prefixing the reStructuredText role target with a ! we disable
conversion to a link, thereby passing continuous integration checks
even though the referenced class is deliberately absent from the
documentation.