Fix the Stream wrapping class. It had moved from elsewhere, but wasn't corrected for its new home in util.py
Fix the Stream wrapping class. It had moved from elsewhere, but wasn't
corrected for its new home in util.py

* util.py:
  - import the string module, but hide it from users
  - Stream.read: drop the 'util.' prefix and use _string
  - Stream.write: drop the 'util.' prefix

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