Python 3.1 support

Here's the patch to support Python >=3.1 with PL/Python.  The
compatibility code is mostly in line with the usual 2->3 C porting
practice and is documented inline.

I needed to create an arguably weird hack to manage the regression
tests.  Instead of creating a new expected file for pretty much every
test file and also for some input files (where Python syntax had
changed), a sed script creates a complete Python 3 compatible set of
input and output files.  Doesn't look pretty but works quite well.  If
anyone has a better idea, please let me know.