Fix #10693: postgresql dialect should reflect DOMAIN types

Fix #10693: postgresql dialect should reflect DOMAIN types

Also, refactor postgresql dialect type reflection into new protected
dialect instance method `PGDialect._reflect_type`

        PGDialect._reflect_type(format_type, enums, domains, type_description)

test #10693:

- psycopg and pg8000 raise different dbapi exceptions when a domain
  constraint fails. Weaken test assertion to catch both

review #10693:
- type_description is required parameter to _reflect_type
- fix docstring typo
- make arrayspec regex a class variable
- Use equality checks on attype_args
- reflected array dimensionality is always 1