bpo-32826: Add "encoding=utf-8" to open() in idle_test/test_help_about. (GH-5639)

bpo-32826: Add "encoding=utf-8" to open() in idle_test/test_help_about. (GH-5639)

GUI test test_file_buttons() only looks at initial ascii-only lines,
but failed on systems where open() defaults to 'ascii' because
readline() internally reads and decodes far enough ahead to encounter
a non-ascii character in CREDITS.txt.
(cherry picked from commit f34e03ec0ea6a4cef8d966087c77e616c4a5893b)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>