Hi,

This patch adds graphics output support for ob-python via matplotlib.
Specifically, it allows to use header argument `:results file
graphics' as follows:

#+begin_src python :file "test.png" :results graphics file
import matplotlib.pyplot as plt
plt.plot([1,2,3,4,5])
#+end_src

The feature is described in the documentation as follows and has been
supported by ob-R, ob-julia, etc.

> ‘graphics’
>      When used along with ‘file’ type, the result is a link to the file
>      specified in ‘:file’ header argument.