[3.11] gh-104018: disallow "z" format specifier in %-format of byte strings (GH-104033) (#104058)

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: GH-104018

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(cherry picked from commit 3ed8c882902a6982fd67e898a5b8a2d619fb5ddf)

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