pip: explicitly require Python 3.6 above (#5083)
pip: explicitly require Python 3.6 above (#5083)

With `python_requires`, we can disallow other versions of Pythons from
installing TensorBoard. Removal in the earlier change was done without
completely understanding its intent.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard