bpo-33962: Use ttk spinbox for IDLE indent space config (GH-22954)

If ttk.Spinbox is not available (Tk < 8.5.9) use readonly ttk.Combobox.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 42d5a4fc)

Co-authored-by: Mark Roseman <mark@markroseman.com>