Update lxml

[encukou]

Updating to lxml 6 results in the followinb change in the and-or lesson:

     <tr>
         <td><code>not</code></td>
-        <td><code>not False</code><br><code>not 5 &lt; 3</code>
-        </td><td>„ne“</td>
-    </tr>
+        <td><code>not False</code><br><code>not 5 &lt; 3
+        <td>„ne“</td>
+    </code></td></tr>

It's be nice to figure out why, fix this, and update.

In the mean time I've pinned lxml<6.