Add Unicode support in PL/Python

PL/Python now accepts Unicode objects where it previously only accepted string
objects (for example, as return value).  Unicode objects are converted to the
PostgreSQL server encoding as necessary.

This change is also necessary for future Python 3 support, which treats all
strings as Unicode objects.

Since this removes the error conditions that the plpython_unicode test file
tested for, the alternative result files are no longer necessary.