implement literal stringification for arrays

implement literal stringification for arrays

as we already implement stringification for the contents,
provide a bracketed syntax for default and ARRAY literal
for PG specifically.   ARRAY literal seems much simpler to
render than their quoted syntax which requires double quotes
for strings.

also open up testing for pg8000 which has likely been
fine with arrays for awhile now, bump the version pin
also.

Fixes: #8138
Change-Id: Id85b052b0a9564d6aa1489160e58b7359f130fdd