Add compatibility properties to Range; implement pep-484

Add compatibility properties to Range; implement pep-484

This adds a bunch of properties to new PG Range class for compatibility
with other implementations, providing a more similar API to access
emptiness and bounds status.

The naming conventions here derive from PostgreSQL itself,
see https://www.postgresql.org/docs/9.3/functions-range.html .

pep-484 also implemented by Mike, as this is a pretty type-intensive
module.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Closes: #8927
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8927
Pull-request-sha: 8b9e7b7e3345673b43aeabd7ec88b88dc3cfa7eb

Change-Id: I0b1d49311517ee1cc1377a974ed0a860ea5756e4