[Cython] pxi or pxd for numpy?

[Brian Granger]

>  What recommendation are you talking about? Using a pxd file
>  to provide a namespace is fine as far as I'm concerned.
>  It's one of the things they were invented for.

Previously, (on this list and at an in person meeting of Sage devs)
some people were saying that .pxi files should be used for this
purpose.  Because of these discussions, numpy.pxd was renamed
numpy.pxi, but now this looks like a bad decision.

>  --
>  Greg
>