Resolve Postgresql implicit indexes via duplicates_constraint

Resolve Postgresql implicit indexes via duplicates_constraint

Fixed bug where autogenerate would produce a DROP statement for the index
implicitly created by a Postgresql EXCLUDE constraint, rather than skipping
it as is the case for indexes implicitly generated by unique constraints.
Makes use of SQLAlchemy 1.0.x's improved "duplicates index" metadata and
requires at least SQLAlchemy version 1.0.x to function correctly.

Change-Id: I7362c8045f69553c6090dd3cb236569b0c9b1e67
Fixes: #461