Fix Python3 errors for cPickle

[bnaul]

Build for Python3 was broken in a9a488d; apparently that was just yesterday but I didn't want to wait :rocket:.

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[bnaul]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

[terrytangyuan]

Thanks!

[terrytangyuan]

@ilblackdragon Could you help cherry pick this one to TF PR?