Issue #15618: Make turtle.py itself work when run from a module with

Issue #15618: Make turtle.py itself work when run from a module with
from __future__ import unicode_literals.  Initial patch by Juancarlo Añez.

The demos at the end of turtle.py appear to be the only test, so I changed
some of the strings to unicode with a u prefix.  If os.path.isfile or the Tk
image function have problems with Unicode input, that would be different issue.