Correctly add exceptions to the plpy module for Python 3

Correctly add exceptions to the plpy module for Python 3

The way the exception types where added to the module was wrong for
Python 3.  Exception classes were not actually available from plpy.
Fix that by factoring out code that is responsible for defining new
Python exceptions and make it work with Python 3.  New regression test
makes sure the plpy module has the expected contents.

Jan Urbanśki, slightly revised by me