:mod:`imaplib` now encodes non-ASCII mailbox names as modified UTF-7
(:rfc:`3501`, section 5.1.3) in the default mode, so international mailbox
names can be passed as ordinary :class:`str`; under ``UTF8=ACCEPT`` they are
sent as UTF-8 instead.  A name that is not valid modified UTF-7, such as one
with a bare ``&``, is now encoded correctly instead of being sent as is.
A ``str`` that is already valid modified UTF-7, or a :class:`bytes` object,
is sent unchanged.
