Log Message:
py-pythran: update to 0.13.0.

* Improve performance of functions revieving scalar arguments. This changes
  the internal function call API.

* Improve performance of fix-stride slicing, using a new slice
  representation.

* Improve numpy.copyto performance, and detect copyto pattern usage.

* Force internal linkage of generated functions, which gives more
  optimization room to the C++ compiler.

* Provide entry points pythran.import_pythrancode and
  pythran.import_pythranfile, as a poor man JIT option.

* Optimize numpy.argmax(cst * val) into numpy.argmax(val) when cst is
  positive.

* Avoid copies upon numpy.array_split

* Get rid of unused functions C++ warnings

* Avoid generating a loop footer when the loop index is not used outside of
  the loop.