Modernize Python exception syntax in tests

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