Refs #27395 -- Added XML namespace declaration to sitemap template.

As per Google recommendations:
https://support.google.com/webmasters/answer/189077?hl=en

django/contrib/sitemaps/templates/sitemap.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 {% spaceless %}
 {% for url in urlset %}
   <url>

tests/sitemaps_tests/test_generic.py

@@ -40,7 +40,7 @@ class GenericViewsSitemapTests(SitemapTestsBase):
         for pk in TestModel.objects.values_list("id", flat=True):
             expected += "<url><loc>%s/testmodel/%s/</loc></url>" % (self.base_url, pk)
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 %s
 </urlset>
 """ % expected
@@ -51,7 +51,7 @@ class GenericViewsSitemapTests(SitemapTestsBase):
         TestModel.objects.update(lastmod=datetime(2013, 3, 13, 10, 0, 0))
         response = self.client.get('/generic-lastmod/sitemap.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>%s/testmodel/%s/</loc><lastmod>2013-03-13</lastmod></url>
 </urlset>
 """ % (self.base_url, test_model.pk)

tests/sitemaps_tests/test_http.py

@@ -67,7 +67,7 @@ class HTTPSitemapTests(SitemapTestsBase):
         "A simple sitemap section can be rendered"
         response = self.client.get('/simple/sitemap-simple.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>%s/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % (self.base_url, date.today())
@@ -92,7 +92,7 @@ class HTTPSitemapTests(SitemapTestsBase):
         "A simple sitemap can be rendered"
         response = self.client.get('/simple/sitemap.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>%s/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % (self.base_url, date.today())
@@ -191,7 +191,7 @@ class HTTPSitemapTests(SitemapTestsBase):
         # doesn't raise an exception.
         response = self.client.get('/simple/sitemap.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>http://testserver/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % date.today()
@@ -257,7 +257,7 @@ class HTTPSitemapTests(SitemapTestsBase):
         "A simple i18n sitemap index can be rendered"
         response = self.client.get('/simple/i18n.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>{0}/en/i18n/testmodel/{1}/</loc><changefreq>never</changefreq><priority>0.5</priority></url><url><loc>{0}/pt/i18n/testmodel/{1}/</loc><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """.format(self.base_url, self.i18n_model.pk)
@@ -266,7 +266,7 @@ class HTTPSitemapTests(SitemapTestsBase):
     def test_sitemap_without_entries(self):
         response = self.client.get('/sitemap-without-entries/sitemap.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 
 </urlset>"""
         self.assertXMLEqual(response.content.decode(), expected_content)

tests/sitemaps_tests/test_https.py

@@ -23,7 +23,7 @@ class HTTPSSitemapTests(SitemapTestsBase):
         "A secure sitemap section can be rendered"
         response = self.client.get('/secure/sitemap-simple.xml')
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>%s/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % (self.base_url, date.today())
@@ -48,7 +48,7 @@ class HTTPSDetectionSitemapTests(SitemapTestsBase):
         "A sitemap section requested in HTTPS is rendered with HTTPS links"
         response = self.client.get('/simple/sitemap-simple.xml', **self.extra)
         expected_content = """<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
 <url><loc>%s/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % (self.base_url.replace('http://', 'https://'), date.today())