Modernize Python exception syntax in tests

Change the exception syntax used in the tests to use the more current

    except Exception as ex:

rather than the old

    except Exception, ex:

Since support for Python <2.6 has been removed, all supported versions
now support the new style, and we can save one step in the Python 3
compatibility conversion.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/98b69261-298c-13d2-f34d-836fd9c29b21%402ndquadrant.com

src/pl/plpython/sql/plpython_params.sql

@@ -29,7 +29,7 @@ CREATE FUNCTION test_param_names3(a0 integer) RETURNS boolean AS $$
 try:
 	assert a1 == args[0]
 	return False
-except NameError, e:
+except NameError as e:
 	assert e.args[0].find("a1") > -1
 	return True
 $$ LANGUAGE plpythonu;