Merged revisions 66686 via svnmerge from

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  r66686 | martin.v.loewis | 2008-09-30 00:09:07 +0200 (Di, 30 Sep 2008) | 5 lines

  Issue #3965: Allow repeated calls to turtle.Screen, by making it a
  true singleton object.

  Reviewed by Gregor Lingl.
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