[issue5670] Speed up pickling of dicts in cPickle

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc <amauryfa at gmail.com> added the comment:

The patch produces different output for an empty dict: a sequence "MARK 
SETITEMS" is written, which is useless and wastes 2 bytes.

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nosy: +amaury.forgeotdarc

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<http://bugs.python.org/issue5670>
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