gh-153319: Read turtledemo scripts with the source file's own encoding (#153321)

The demo viewer read each script with open() and no encoding argument, i.e. the locale default encoding. That emits an EncodingWarning under -X warn_default_encoding and can mis-decode a script on a non-UTF-8 locale. Read the source with tokenize.open() instead, which decodes using the encoding detected from the file (its coding cookie, else UTF-8), matching how Python read the module when it was imported just above.

* gh-153319: Read turtledemo scripts as UTF-8

Per review, open the demo scripts with encoding='utf-8' instead of
tokenize.open(); the scripts are UTF-8.

---------

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/turtledemo/__main__.py

@@ -333,7 +333,7 @@ class DemoWindow(object):
         modname = 'turtledemo.' + filename
         __import__(modname)
         self.module = sys.modules[modname]
-        with open(self.module.__file__, 'r') as f:
+        with open(self.module.__file__, encoding='utf-8') as f:
             chars = f.read()
         self.text.delete("1.0", "end")
         self.text.insert("1.0", chars)

Misc/NEWS.d/next/Library/2026-07-08-19-40-00.gh-issue-153319.Qk7bE2.rst

@@ -0,0 +1,2 @@
+The :mod:`turtledemo` viewer now reads each demo script as UTF-8 instead of
+the locale encoding.