title: Don't understand why this AttributeError: 'Graph' object has no attribute 'merge_one' is occurring
body: <p>I'm following a tutorial, and I'm using the last python2 (homebrew) with PyCharm (with project interpreter configured) - But I'm stuck in this part:</p>
  
  <pre><code>from py2neo import Graph, Node
  graph = Graph()
  nicole = Node("Person", name="Nicole")
  graph.create(nicole)
  graph.delete(nicole)
  nicole = graph.merge_one("Person", "name", "Nicole")
  </code></pre>
  
  <p>Error:</p>
  
  <pre class="lang-none prettyprint-override"><code>Traceback (most recent call last):
      File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  AttributeError: 'Graph' object has no attribute 'merge_one'
  </code></pre>
  
  <p>I already checked the <a href="https://py2neo.org/2.0/intro.html?highlight=merge" rel="nofollow noreferrer">documentation</a> and it seems that I'm doing everything ok . I tried to uninstall and install last version of py2neo but with no success. How I solve this problem?</p>
  
  <p>Expected behavior: Running that command from the python2 console: If that Person exists don't duplicate it but change its values, if don't exist create it.</p>