title: lxml relative xPath doesn't return result relative to the given HtmlElement
body: <p>I apply a relative XPath (<code>./</code>) to an HtmlElement and it doesn't return any results. When I try using double dots (<code>../</code>), it returns all results matching from root HTML instead of descendant results of that specific HtmlElement. I am not sure what is wrong here.</p>
  <p>The version of lxml is 4.5.2</p>
  <p>Example:</p>
  <pre><code>&lt;html&gt;
    &lt;h3&gt;
    &lt;p&gt;
      &lt;table&gt;
        &lt;tr&gt;
          &lt;td&gt;Sample&lt;/td&gt;
          &lt;td&gt;Sample&lt;/td&gt;
        &lt;/tr&gt;
      &lt;/table&gt;
    &lt;/p&gt;
    &lt;h3&gt;
    &lt;p&gt;
      &lt;table&gt;
        &lt;tr&gt;
          &lt;td&gt;Sample 2&lt;/td&gt;
          &lt;td&gt;Sample 2&lt;/td&gt;
        &lt;/tr&gt;
      &lt;/table&gt;
    &lt;/p&gt;
  &lt;/html&gt;
  </code></pre>
  <p>Code</p>
  <pre><code>r = requests.get('http://website.com')
  tree = html.fromstring(r.content)
  tables = tree.xpath(&quot;(//p/table)&quot;)
  
  for table in tables:
      result = table.xpath('.//td')
      text = result.text_content()
  </code></pre>
  <p>The first iteration in the loop should return &quot;Sample&quot; texts and the second iteration should return &quot;Sample 2&quot; texts.</p>