now async becomes keyword!

gistfile1.txt

[1] tkomiya@deneb> python3.6
Python 3.6.5 (default, Mar 31 2018, 01:44:52)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo(**kwargs):
...     return kwargs
...
>>> foo(async=True)
{'async': True}
>>>
tkomiya@deneb> python3.7
Python 3.7.0b3 (default, May 20 2018, 22:05:04)
[Clang 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo(**kwargs):
...     return kwargs
...
>>> foo(async=True)
  File "<stdin>", line 1
    foo(async=True)
            ^
SyntaxError: invalid syntax
>>>