Allow access to the parser during the read phase

sphinx/builders/__init__.py

@@ -485,6 +485,7 @@ def read_doc(self, docname: str, *, _cache: bool = True) -> None:
         filename = self.env.doc2path(docname)
         filetype = get_filetype(self.app.config.source_suffix, filename)
         publisher = self.app.registry.get_publisher(self.app, filetype)
+        self.env.temp_data['parser'] = publisher.parser
         # record_dependencies is mutable even though it is in settings,
         # explicitly re-initialise for each document
         publisher.settings.record_dependencies = DependencyList()