[3.13] gh-129873: IDLE: Improve help.py's method of parsing HTML (GH-129859) (#129884)

gh-129873: IDLE: Improve help.py's method of parsing HTML (GH-129859)

In `help.copy_strip`, only copy the text `<section>`.  In `help.HelpParser.handle_starttag` and elsewhere, remove code to skip the no longer present html.  Add a reminder at the top of idle.rst to run copy_strip after changes.
---------

(cherry picked from commit 6fbf15f98e04f582aeccf5334a94840149ff7cd5)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>