Enable autoDocstring for pyx file extension

[leonardo-vinci]

Hi,

Currently I think it is not possible to use autoDocstring in .pyx files. I have a project in Cython and use Cython code. In order to use autodocstring whenever I need it I rename the file to .py and then rename it back to .pyx.

I understand that there is not much needed from autoDocstring extension except to enable it to work with .pyx files. I suggest this file extension is added to autoDocstring.