[issue29659] Expose the `length` arg from shutil.copyfileobj for public use

[Terry J. Reedy]

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


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nosy: +terry.reedy
stage:  -> test needed
type:  -> enhancement

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29659>
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