#9124: mailbox now accepts binary input and uses binary internally

Although this patch contains API changes and is rather weighty for an
RC phase, the mailbox module was essentially unusable without the patch
since it would produce UnicodeErrors when handling non-ascii input
at arbitrary and somewhat mysterious places, and any non-trivial amount
of email processing will encounter messages with non-ascii bytes.
The release manager approved the patch application.

The changes allow binary input, and reject non-ASCII string input early
with a useful message instead of failing mysteriously later.  Binary
is used internally for reading and writing the mailbox files.  StringIO
and Text file input are deprecated.

Initial patch by Victor Stinner, validated and expanded by R. David Murray.