Issue #26355: Specify canonical URLs in docs pages

Add canonical header link on each page to corresponding major
version of the documentation.

Patch by Matthias Bussonnier.

Doc/tools/templates/layout.html

@@ -38,6 +38,7 @@
 {% endblock %}
 {% block extrahead %}
     <link rel="shortcut icon" type="image/png" href="{{ pathto('_static/py.png', 1) }}" />
+    <link rel="canonical" href="https://docs.python.org/3/{{pagename}}.html" />
     {% if builder != "htmlhelp" %}
     {% if not embedded %}<script type="text/javascript" src="{{ pathto('_static/copybutton.js', 1) }}"></script>{% endif %}
     {% if versionswitcher is defined and not embedded %}<script type="text/javascript" src="{{ pathto('_static/version_switch.js', 1) }}"></script>{% endif %}

Misc/ACKS

@@ -218,6 +218,7 @@ Alastair Burt
 Tarn Weisner Burton
 Lee Busby
 Katherine Busch
+Matthias Bussonnier
 Ralph Butler
 Laurent De Buyst
 Zach Byrne

Misc/NEWS

@@ -70,6 +70,9 @@ C API
 Documentation
 -------------
 
+- Issue #26355: Add canonical header link on each page to corresponding major
+  version of the documentation. Patch by Matthias Bussonnier.
+
 - Issue #29349: Fix Python 2 syntax in code for building the documentation.