async is a reserved word: target.cuda(non_blocking=True)

[flake8](http://flake8.pycqa.org) testing of https://github.com/jantic/DeOldify on Python 3.7.1

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./fastai/core.py:37:32: E999 SyntaxError: invalid syntax
    if cuda: a = to_gpu(a, async=True)
                               ^
./fastai/models/cifar10/main_dxy.py:179:32: E999 SyntaxError: invalid syntax
      target = target.cuda(async=True)
                               ^
2     E999 SyntaxError: invalid syntax
2
```

fastai/models/cifar10/main_dxy.py

@@ -176,7 +176,7 @@ def train(train_loader, model, criterion, optimizer, epoch, log):
     data_time.update(time.time() - end)
 
     if args.use_cuda:
-      target = target.cuda(async=True)
+      target = target.cuda(non_blocking=True)
       input = input.cuda()
     input_var = torch.autograd.Variable(input)
     target_var = torch.autograd.Variable(target)