Add Python porting howto from 3.x.

No change was necessary to make the file build, contrary to cporting.rst
that uses markup like :c:func: that needs to be changed to :cfunc: in
2.7.  Future backports should be straightforward.
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index 09bc5cb..94ecc9a 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -14,6 +14,7 @@
    :maxdepth: 1
 
    advocacy.rst
+   pyporting.rst
    cporting.rst
    curses.rst
    descriptor.rst