Legacy Python2 and early Python3 leftovers

[vadimkantorov]

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

[vadimkantorov]

Are python-enum and python-six third_party dependencies still needed?

[ezyang]

These should all be doable. python-enum/python-six should be droppable, but it looks like it came from nnpack so we would to make sure they've gone Python 3 only, or at least it's an optional dependency.

[lithomas1]

@ezyang It seems like a lot of legacy Python 2 code is in the caffe2 module. Are you still taking PRs for that section of the code even though it is deprecated?

[ezyang]

No, it's not worth it