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/expected/plpython_spi.out

@@ -125,7 +125,7 @@ else:
    return None
 $$ LANGUAGE plpythonu;
 SELECT result_nrows_test();
-INFO:  (True,)
+INFO:  True
 CONTEXT:  PL/Python function "result_nrows_test"
  result_nrows_test 
 -------------------