exceptions not present in plpy with Python 3

Hi,

there seems to be a problem in the way we add exceptions to the plpy
module in PL/Python compiled with Python 3k.

Try this: DO $$ plpy.SPIError $$ language plpython3u;

I'm not a Python 3 expert, but I nicked some code from the Internet and
came up with this patch (passes regression tests on both Python 2 and 3).

The funny thing is that it never blew up earlier, because we only used
plpy.SPIError in except: blocks that weren't even executed, as errors in
plpy.execute just terminate the function.

With my changes they turn into catchable exceptions, and so accessing
plpy.SPIError in Python 3 becomes essential.

BTW: do we have any buildfarm animal that uses Python 3?

Cheers,
Jan