Support DROP CONSTRAINT IF EXISTS

Support DROP CONSTRAINT IF EXISTS

Added :paramref:`.op.drop_constraint.if_exists` parameter to
:func:`.op.drop_constraint` which will render "DROP CONSTRAINT IF EXISTS".
Pull request courtesy Aaron Griffin.

Also attempting to fix unrelated issue w/ test_stubs and python
versions which may be in a separate patch if it can't work here

Fixes: #1650
Closes: #1651
Pull-request: https://github.com/sqlalchemy/alembic/pull/1651
Pull-request-sha: 1c3c0d388d36fa60b9aaa013873a415488ed7796

Change-Id: I96378d45dd00898975f450299e6f1344bb65ccff