Reflect unique constraints when reflecting a Table object

Reflect unique constraints when reflecting a Table object

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.