[issue17814] Popen.stdin/stdout/stderr documentation should mention object type

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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versions:  -Python 3.1, Python 3.2, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17814>
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