`async` is a keyword in Python 3.7

`async` has become a keyword in Python 3.7 and can not be used as an argument name . See https://mail.python.org/pipermail/python-dev/2017-March/147678.html

```
Python 3.7.0a3 (v3.7.0a3:90a6785, Dec 5 2017, 22:50:42) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pycuda.gpuarray as gpuarray
Traceback (most recent call last):
File "", line 1, in
File "X:\Python37\lib\site-packages\pycuda\gpuarray.py", line 230
def set(self, ary, async=False, stream=None):
^
SyntaxError: invalid syntax
```

Also affects PyOpenCL.