text: Title: UniqueConstraint not working with SQLAlchemy
  Body: Hi folks,
  
  I seem to be having issues with `UniqueConstraint`s.
  
  In models I have:
  
  ```py
  class SubjectCategory(Base):
      __tablename__ = 'subject_categories'
  
      id = Column(Integer, primary_key=True, nullable=False)
      name = Column(String(63))
      region = Column(Integer, ForeignKey('regions.id'), nullable=False)
  
      __tableargs__ = UniqueConstraint('name', 'region', name='_subject_category_uc')
  ```
  That should create a UniqueConstraint on `name` and `region` together right?
  
  However, when I look at the database:
  ```
  my_database=# \d subject_categories
                  Table "public.subject_categories"
   Column |         Type          | Collation | Nullable | Default 
  --------+-----------------------+-----------+----------+---------
   id     | integer               |           | not null | 
   name   | character varying(63) |           |          | 
   region | integer               |           | not null | 
  Indexes:
      "subject_categories_pkey" PRIMARY KEY, btree (id)
  Foreign-key constraints:
      "subject_categories_region_fkey" FOREIGN KEY (region) REFERENCES regions(id)
  Referenced by:
      TABLE "subjects" CONSTRAINT "subjects_category_fkey" FOREIGN KEY (category) REFERENCES subject_categories(id)
  ```
  I have no constraint.
  
  Have I implemented this wrongly?
  
  Thanks,
  Tom