make opening brace of container literals and comprehensions correspond to the line number and col offset of the AST node (closes #25131)

This associates the message box with the widget and is better for Mac OSX.
Patch by Mark Roseman.

Some of the calls to the Frm class were buggy anyway.

The height is reduced by 3/4 to account for extra spacing between lines,
relative to an Idle editor, and extra tall header lines.

by adding a new tag.  Patch by Mark Roseman. Also give <pre> text a very
light blueish-gray background similar to that used by Sphinx html.

Patch by Benjamin Hodgson.

Patch written by Prashant Tyagi.

to finish making current README.txt obsolete.

Some were missing and notes were not correct.

Some of the details of comparing mixed types were incorrect or ambiguous.
NotImplemented is only relevant at a lower level than the Expressions
chapter. Added details of comparing range() objects, and default behaviour
and consistency suggestions for user-defined classes. Patch from Andy Maier.

so it is not triggered on import.  The problem is creation of a now-unused
instance "helpDialog = HelpDialog()", left for back compatibility.
So instead trigger the warning when that instance or another is used.

This restores the ability to write encoding names in uppercase like "UTF-8",
which worked in Python 2.