Remove (most) Python 2 support from Python code (#35615)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35615

Python 2 has reached end-of-life and is no longer supported by PyTorch.
Now we can clean up a lot of cruft that we put in place to support it.
These changes were all done manually, and I skipped anything that seemed
like it would take more than a few seconds, so I think it makes sense to
review it manually as well (though using side-by-side view and ignoring
whitespace change might be helpful).

Test Plan: CI

Differential Revision: D20842886

Pulled By: dreiss

fbshipit-source-id: 8cad4e87c45895e7ce3938a88e61157a79504aed

test/test_namedtensor.py

@@ -6,7 +6,6 @@ import itertools
 import functools
 import torch
 from torch import Tensor
-from torch._six import PY2
 import torch.nn.functional as F
 from multiprocessing.reduction import ForkingPickler
 import pickle
@@ -162,7 +161,6 @@ class TestNamedTensor(TestCase):
         self.assertTrue(partially_named.has_names())
         self.assertTrue(fully_named.has_names())
 
-    @unittest.skipIf(PY2, "Ellipsis object not supported in python 2")
     def test_py3_ellipsis(self):
         # Need to exec or else flake8 will complain about invalid python 2.
         tensor = torch.randn(2, 3, 5, 7)