Render parenthesis around sqlite expression defaults

Render parenthesis around sqlite expression defaults

Fixed bug in SQLite DDL where using an expression as a server side default
required that it be contained within parenthesis to be accepted by the
sqlite parser.  Pull request courtesy Bartlomiej Biernacki.

Fixes: #4474
Closes: #4475
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4475
Pull-request-sha: 0af3238c69c2610333cf62082c12047d45c31ce0

Change-Id: I888c69bfba5a5ab8ffa420f512557ac311a36b31
(cherry picked from commit f7ce37e9dd493d4b60b3156f08ef4c98fd63904d)