swig-py: Add close to the core.Stream class.

* subversion/bindings/swig/python/svn/core.py
  (Stream.read, Stream.write): Raise a ValueError when the stream isn't open.
  (Stream.close): New function.

Patch by: Alexey Neyman <stilor{_at_}att.net>


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619077 13f79535-47bb-0310-9956-ffa450edef68