Make setup.py Python-2 syntactically correct (#41960)
Summary:
Import __future__ to make `print(*args)` a syntactically correct statement under Python-2
Otherwise, if once accidentally invokes setup.py using Python-2 interpreter they will be greeted by:
```
File "setup.py", line 229
print(*args)
^
SyntaxError: invalid syntax
```
instead of:
```
Python 2 has reached end-of-life and is no longer supported by PyTorch.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41960
Reviewed By: orionr, seemethere
Differential Revision: D22710174
Pulled By: malfet
fbshipit-source-id: ffde3ddd585707ba1d39e57e0c6bc9c4c53f8004
