Tweak tests to support Python 3.7

Python 3.7 removes the trailing comma in the repr() of
BaseException (see <https://bugs.python.org/issue30399>), leading to
test output differences.  Work around that by composing the equivalent
test output in a more manual way.

src/pl/plpython/sql/plpython_subtransaction.sql

@@ -80,7 +80,7 @@ with plpy.subtransaction():
     except plpy.SPIError, e:
         if not swallow:
             raise
-        plpy.notice("Swallowed %r" % e)
+        plpy.notice("Swallowed %s(%r)" % (e.__class__.__name__, e.args[0]))
 return "ok"
 $$ LANGUAGE plpythonu;