gh-104018: disallow "z" format specifier in %-format of byte strings (GH-104033)

PEP-0682 specified that %-formatting would not support the "z" specifier,
but it was unintentionally allowed for bytes. This PR makes use of the "z"
flag an error for %-formatting in a bytestring.

Issue: #104018

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>