Does not generate correct output with Python 3.13

[s-t-e-v-e-n-k]

Running the testsuite against Python 3.13 causes widespread devastation, which I have managed to nail down to the app not actually firing under Python 3.13. Comparing generated HTML files from the test_buildhtml_alt testcase, since it's one of the more trivial ones between Python 3.12 and 3.13 gives us:

--- /var/tmp/build-root/openSUSE_Tumbleweed-x86_64/tmp/12.html  2025-01-24 13:01:51.175671622 +1100
+++ /var/tmp/build-root/openSUSE_Tumbleweed-x86_64/tmp/13.html  2025-01-24 13:01:53.144741669 +1100
@@ -32,9 +32,9 @@
         <div class="bodywrapper">
           <div class="body" role="main">

-  <p class="plantuml">
-<img src="_images/plantuml-e439fab77189cc1ec7958d2d93446374fb00345d.png" alt="Foo &lt;Bar&gt;"/>
-</p>
+  <p>ml::
+alt: Foo &lt;Bar&gt;</p>
+<p>ello</p>


             <div class="clearer"></div>

Which certainly seems like the uml:: directive is not expanded. I'm happy to help debug this further if required.