Fix sphinx-doc#8915: html theme: The translation of sphinx_rtd_theme …

…does not work

Since sphinx_rtd_theme-0.5.0, it supports translations.  But Sphinx core
disallows to enable it because theming framework gives special treatment
for the theme for a long time.

This goes to load it via setuptools at first to enable the translations.

Note: The special treatment for sphinx_rtd_theme (< 0.2.0) is not
removed yet.  But it will be removed in the next major release.

CHANGES

@@ -61,6 +61,7 @@ Features added
   :confval:`gettext_additional_targets` setting)
 * #2018: html: :confval:`html_favicon` and :confval:`html_logo` now accept URL
   for the image
+* #8915: html theme: The translation of sphinx_rtd_theme does not work
 * #8070: html search: Support searching for 2characters word
 * #7830: Add debug logs for change detection of sources and templates
 * #8201: Emit a warning if toctree contains duplicated entries

sphinx/theming.py

@@ -178,8 +178,6 @@ def load_extra_theme(self, name: str) -> None:
         """Try to load a theme having specifed name."""
         if name == 'alabaster':
             self.load_alabaster_theme()
-        elif name == 'sphinx_rtd_theme':
-            self.load_sphinx_rtd_theme()
         else:
             self.load_external_theme(name)
 
@@ -237,13 +235,13 @@ def create(self, name: str) -> Theme:
         if name not in self.themes:
             self.load_extra_theme(name)
 
+        if name not in self.themes and name == 'sphinx_rtd_theme':
+            # sphinx_rtd_theme (< 0.2.0)  # RemovedInSphinx50Warning
+            logger.warning(__('sphinx_rtd_theme (< 0.2.0) found. '
+                              'It will not be available since Sphinx-5.0'))
+            self.load_sphinx_rtd_theme()
+
         if name not in self.themes:
-            if name == 'sphinx_rtd_theme':
-                raise ThemeError(__('sphinx_rtd_theme is no longer a hard dependency '
-                                    'since version 1.4.0. Please install it manually.'
-                                    '(pip install sphinx_rtd_theme)'))
-            else:
-                raise ThemeError(__('no theme named %r found '
-                                    '(missing theme.conf?)') % name)
+            raise ThemeError(__('no theme named %r found (missing theme.conf?)') % name)
 
         return Theme(name, self.themes[name], factory=self)