Remove obsolete HTML keyword link rel="shortcut" (#11069)

The link relationship keyword `shortcut` does not appear in the HTML 5 specification [1].

It was used by historic browsers (i.e. Internet Explorer 6) which Sphinx no longer supports.

[1]: HTML5 Specification, 4.6.7.8 Link type "icon", https://html.spec.whatwg.org/#rel-icon

sphinx/themes/basic/layout.html

@@ -137,7 +137,7 @@ <h3>{{ _('Navigation') }}</h3>
           href="{{ pathto('_static/opensearch.xml', 1) }}"/>
     {%- endif %}
     {%- if favicon_url %}
-    <link rel="shortcut icon" href="{{ favicon_url|e }}"/>
+    <link rel="icon" href="{{ favicon_url|e }}"/>
     {%- endif %}
     {%- endif %}
 {%- block linktags %}

tests/test_build_html.py

@@ -1370,7 +1370,7 @@ def test_html_remote_logo(app, status, warning):
 
     result = (app.outdir / 'index.html').read_text(encoding='utf8')
     assert ('<img class="logo" src="https://www.python.org/static/img/python-logo.png" alt="Logo"/>' in result)
-    assert ('<link rel="shortcut icon" href="https://www.python.org/static/favicon.ico"/>' in result)
+    assert ('<link rel="icon" href="https://www.python.org/static/favicon.ico"/>' in result)
     assert not (app.outdir / 'python-logo.png').exists()