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 f34e03ec0e)
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Split TextViewer class into ViewWindow, ViewFrame, and TextFrame classes so that instances
of the latter two can be placed with other widgets within a multiframe window.
Patch by Cheryl Sabella.
