Fix test suite under Sphinx 7.4+

Closes breathe-doc#987.

Author: yut23

tests/test_renderer.py

@@ -167,13 +167,14 @@ def mask(self, node):
 
 class MockContext:
     def __init__(self, app, node_stack, domain=None, options=[]):
+        from docutils.statemachine import StringList
         self.domain = domain
         self.node_stack = node_stack
         self.directive_args = [
             None,  # name
             None,  # arguments
             options,  # options
-            None,  # content
+            StringList([], items=[]),  # content
             None,  # lineno
             None,  # content_offset
             None,  # block_text