Legacy Python2 and early Python3 leftovers

1. A lot of references to Python 2 and workarounds: https://github.com/pytorch/pytorch/blob/master/torch/tensor.py
2. L631:
```
        if array.dtype == bool:
            # Workaround, torch has no built-in bool tensor
            array = array.astype('uint8')
```
But PyTorch has built-in bool tensor since a long while

cc @ezyang