turtledemo example viewer decodes scripts with the locale default encoding

Bug report

The turtledemo example viewer opens each demo script for display with
open(self.module.__file__, 'r') in DemoWindow.loadfile
(Lib/turtledemo/__main__.py). No encoding argument is passed, so the
source is decoded with the locale default encoding.

This emits an EncodingWarning under -X warn_default_encoding, and on a
non-UTF-8 locale (for example Windows cp1252) it can mis-decode a demo whose
source is UTF-8 or carries a PEP 263 coding cookie, showing mojibake in the
viewer.

It is the only unspecified-encoding open() in Lib/turtledemo.

Reproduction

Launching the viewer under -X warn_default_encoding and loading any example
reaches open(self.module.__file__, 'r') in DemoWindow.loadfile, which
raises under -W error::EncodingWarning:

EncodingWarning: 'encoding' argument not specified

On a non-UTF-8 locale the same call decodes the source with the wrong codec.

Fix

Read the source with tokenize.open(), which decodes using the encoding
detected from the file (its PEP 263 coding cookie, else UTF-8). That is the
correct encoding for reading Python source, and it matches how the module was
just imported one line above.

Linked PRs

• gh-153321

Inizia in Lib/turtledemo/main.py, in DemoWindow.loadfile, dove il codice sorgente della demo viene aperto per essere visualizzato. Verifica che il visualizzatore utilizzi la codifica del sorgente rilevata dal file e verifica che il caricamento degli esempi non emetta più un EncodingWarning con -X warn_default_encoding.