title: Add CheckConstraint through __table_args__ dict
body: <p>I have a model with <code>__table_args__</code> as a dict:</p>
  
  <pre class="lang-py prettyprint-override"><code>class User(DeclarativeBase):
      __tablename__ = "users"
      __table_args__ = {
          "mysql_charset": "utf8",
          "mysql_collate": "utf8_general_ci",
      }
  
      # ...
  </code></pre>
  
  <p>How can I add a <code>CheckConstraint</code> to this <code>__table_args__</code> dict ? Usually and <a href="https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/table_config.html#table-configuration" rel="nofollow noreferrer">sqlalchemy doc</a> use a <code>tuple</code> to show how add <code>CheckConstraint</code> to models.</p>