Remove link elements in builders other than html

This change fixes EPUBs

Author: m-aciek

Doc/tools/templates/layout.html

@@ -29,7 +29,7 @@
     {% if builder == "html" and enable_analytics %}
       <script defer data-domain="docs.python.org" src="https://analytics.python.org/js/script.outbound-links.js"></script>
     {% endif %}
-    <link rel="canonical" href="https://docs.python.org/3/{{pagename}}.html">
+    {% if builder == "html" %}<link rel="canonical" href="https://docs.python.org/3/{{pagename}}.html">{% endif %}
     {% if builder != "htmlhelp" %}
       {% if pagename == 'whatsnew/changelog' and not embedded %}
       <script type="text/javascript" src="{{ pathto('_static/changelog_search.js', 1) }}"></script>{% endif %}