Message 112165 - Python tracker

Patch updated here, and on Rietveld too.
http://codereview.appspot.com/664043

Rules (as discussed):
 - tree.tostring(encoding=None)  => encodes to "US-ASCII"
   (compatible with 2.7 and lxml.etree)
 - tree.tostring(encoding="unicode") => outputs Unicode
 - tree.tostring(encoding=str) => outputs Unicode
   (compatible with lxml.etree)

For 2.7, no change planned.
For 3.1, do we keep the current behavior?
  - tree.tostring(encoding=None)  => outputs Unicode