Fix translator_setup: undefined default_translator_class for linkcheck builder (#2029)

The translator setup is called unconditionally to add the
HootstrapHTML5TransatorMixin. If no translators have been set explicitly
this falls back to `app.builder.default_translator_class`, which is
however not defined for the `linkcheck` builder (maybe others),
resulting in an attribute error:
```console
$ sphinx-build -T -b linkcheck <src> <dest>
<...>
Traceback (most recent call last):
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/events.py", line 404, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/PATH/TO/venv/lib/python3.11/site-packages/pydata_sphinx_theme/translator.py", line 88, in setup_translators
    app.builder.default_translator_class,
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'CheckExternalLinksBuilder' object has no attribute 'default_translator_class'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/cmd/build.py", line 496, in build_main
    app = Sphinx(
          ^^^^^^^
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/application.py", line 295, in __init__
    self._init_builder()
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/application.py", line 369, in _init_builder
    self.events.emit('builder-inited')
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/events.py", line 415, in emit
    raise ExtensionError(
sphinx.errors.ExtensionError: Handler <function setup_translators at 0x7f73ec330400> for event 'builder-inited' threw an exception (exception: 'CheckExternalLinksBuilder' object has no attribute 'default_translator_class')
```

This PR adds a safe check for `app.builder.default_translator_class` and
skips the translator setup if the attribute is not defined.

I have tested the modified version with `linkcheck`, but I am not sure
where to add a unit test for these changes.

Author: lang-m

src/pydata_sphinx_theme/translator.py

@@ -76,11 +76,17 @@ def setup_translators(app: Sphinx):
     If we don't detect an HTML-based translator, then we do nothing.
     """
     if not app.registry.translators.items():
+        try:
+            default_translator_class = app.builder.default_translator_class
+        except AttributeError:
+            # some builders, e.g. linkcheck, do not define 'default_translator_class'
+            return
+
         translator = types.new_class(
             "BootstrapHTML5Translator",
             (
                 BootstrapHTML5TranslatorMixin,
-                app.builder.default_translator_class,
+                default_translator_class,
             ),
             {},
         )