Add compiler support for PostgreSQL "NOT VALID" constraints.
Added compiler support for the PostgreSQL ``NOT VALID`` phrase when rendering
DDL for the :class:`.CheckConstraint`, :class:`.ForeignKeyConstraint`
and :class:`.ForeignKey` schema constructs. Pull request courtesy
Gilbert Gilb's.
Fixes: #7600
Closes: #7601
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7601
Pull-request-sha:
78eecd55fd9fad07030d963f5fd6713c4af60e80
Change-Id: I84bfe84596856eeea2bcca45c04ad23d980a75ec
(cherry picked from commit
77dd6808f250e0431f9bce824f46f6e1ef63eef3)