bpo-34538: Remove Exception subclassing from tutorial (GH-30361)

Remove the bit about subclassing exceptions.

Documentation PR can skip the NEWS label.

Automerge-Triggered-By: GH:iritkatriel
(cherry picked from commit 2db56130631255ca2eb504519430fb2f1fe789e9)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>