Issue #14777: In an X11 windowing environment, tkinter may return

Issue #14777: In an X11 windowing environment, tkinter may return
undecoded UTF-8 bytes as a string when accessing the Tk clipboard.
Modify clipboad_get() to first request type UTF8_STRING when no
specific type is requested in an X11 windowing environment, falling
back to the current default type STRING if that fails.
Original patch by Thomas Kluyver.