Improve PL/Python elog output

When the elog functions (plpy.info etc.) get a single argument, just print
that argument instead of printing the single-member tuple like ('foo',).

src/pl/plpython/sql/plpython_test.sql

@@ -27,6 +27,7 @@ plpy.debug('debug')
 plpy.log('log')
 plpy.info('info')
 plpy.info(37)
+plpy.info()
 plpy.info('info', 37, [1, 2, 3])
 plpy.notice('notice')
 plpy.warning('warning')