Single-argument functions cannot be called using keyword

```
In [1]: %load_ext cython

In [2]: %%cython
   ...: def f(k): return k

In [3]: f(k=1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-2f40ef43e783> in <module>()
----> 1 f(k=1)

TypeError: f() takes no keyword arguments
```