bpo-38022: IDLE: upgrade help.html to sphinx 2.x HTML5 output (GH-15664)

The HTML5 output from Sphinx 2.x adds  '<p>' tags within list elements.  Using a new prevtag attribute, ignore these instead of emitting unwanted '\n\n'.

Also stop looking for 'first' classes on tags (no longer present) and fix the bug of double-spacing instead of single spacing after <pre> blocks.