title: setting neo4j uniqueness constraints in py2neo v3
body: <p>Using v2 py2neo I could put this in <code>__init__.py</code></p>
  
  <pre><code>graph.cypher.execute("CREATE CONSTRAINT ON (n:User) ASSERT n.username IS UNIQUE")
  </code></pre>
  
  <p>Why does v3 py2neo</p>
  
  <pre><code>graph.run("CREATE CONSTRAINT ON (n:User) ASSERT n.username IS UNIQUE")
  </code></pre>
  
  <p>fail with this error?</p>
  
  <blockquote>
    <p>TypeError: unbound method run() must be called with Graph instance as
    first argument (got str instance instead)</p>
  </blockquote>