PGDialect.get_check_constraints: Handle "NOT VALID"

PGDialect.get_check_constraints: Handle "NOT VALID"

Added support for reflection of CHECK constraints that include the special
PostgreSQL qualifier "NOT VALID", which can be present for CHECK
constraints that were added to an exsiting table with the directive that
they not be applied to existing data in the table. The PostgreSQL
dictionary for CHECK constraints as returned by
:meth:`.Inspector.get_check_constraints` may include an additional entry
``dialect_options`` which within will contain an entry ``"not_valid":
True`` if this symbol is detected.   Pull request courtesy Bill Finn.

Fixes: #4824
Closes: #4825
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4825
Pull-request-sha: a0e1ab133c2d46521a74e55423ac2ba866682dae

Change-Id: I78365f50055c95474c92124b85df66c5c80c00c8