Use preloaded for sql.util import in exc

Use preloaded for sql.util import in exc

Repaired a function-level import that was not using SQLAlchemy's standard
late-import system within the sqlalchemy.exc module.

Moves 1.3 dependencies() to sqlalchemy.util.preloaded so that it
does not depend on langhelpers which depends on exc.

Fixes: #5632
Change-Id: I61b7ce9cd461071ce543714739f67aa5aeb47fd6
(cherry picked with major changes from commit 2f28ba5513fc7da4efb315e1a03d50ebe40ca566)