at the engine level is not the same thing as at the connect args
level.

- add a test for PG reflection of unique index without any unique
constraint
- for PG, don't include 'duplicates_constraint' in the entry
if the index does not actually mirror a constraint
- use a distinct method for unique constraint reflection within table
- catch unique constraint not implemented condition; this may
be within some dialects and also is expected to be supported by
Alembic tests
- migration + changelogs for #3184
- add individual doc notes as well to MySQL, Postgreql
fixes #3184

psycopg2's ``.closed`` accessor is not as reliable as we assumed,
so we have added an explicit check for the exception messages
"SSL SYSCALL error: Bad file descriptor" and
"SSL SYSCALL error: EOF detected" when detecting an
is-disconnect scenario.   We will continue to consult psycopg2's
connection.closed as a first check.
fixes #3021

Calls to reflect a table did not create any UniqueConstraint objects.
The reflection core made no calls to get_unique_constraints and as
a result, the sqlite dialect would never reflect any unique constraints.

MySQL transparently converts unique constraints into unique indexes, but
SQLAlchemy would reflect those as an Index object and as a
UniqueConstraint. The reflection core will now deduplicate the unique
constraints.

PostgreSQL would reflect unique constraints as an Index object and as
a UniqueConstraint object. The reflection core will now deduplicate
the unique indexes.

- repair get_view_names()
- changelog + migration note