Execute `jupyter nbconvert` on python3 instead of python2.
Execute `jupyter nbconvert` on python3 instead of python2.

python2 fails with:
`RuntimeError: Kernel died before replying to kernel_info`

python3 seems to pass.

PiperOrigin-RevId: 272718077

tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor