Fix compatibility with MyST-Parser (#14194)

Author: AA-Turner

CHANGES.rst

@@ -7,6 +7,8 @@ Dependencies
 * #14153: Drop Python 3.11 support.
 * #12555: Drop Docutils 0.20 support.
   Patch by Adam Turner
+* #13713: Fix compatibility with MyST-Parser.
+  Patch by Adam Turner
 
 Incompatible changes
 --------------------

sphinx/io.py

@@ -32,7 +32,7 @@
 warnings.warn('sphinx.io is deprecated', RemovedInSphinx10Warning, stacklevel=2)
 
 
-class SphinxBaseReader(standalone.Reader['Any']):
+class SphinxBaseReader(standalone.Reader):  # type: ignore[type-arg]
     """A base class of readers for Sphinx.
 
     This replaces reporter by Sphinx's on generating document.

sphinx/util/docutils.py

@@ -12,6 +12,7 @@
 
 import docutils
 import docutils.frontend
+import docutils.writers
 from docutils import nodes
 from docutils.io import FileOutput
 from docutils.parsers.rst import Directive, directives, roles
@@ -879,6 +880,8 @@ def _parse_str_to_doctree(
     transformer.add_transforms(_READER_TRANSFORMS)
     transformer.add_transforms(transforms)
     transformer.add_transforms(parser.get_transforms())
+    # https://github.com/sphinx-doc/sphinx/issues/13713
+    transformer.components['writer'] = _DummyWriter()  # type: ignore[index]
 
     if default_role:
         default_role_cm = rst.default_role(env.current_document.docname, default_role)
@@ -927,6 +930,11 @@ def _get_settings(
     return settings
 
 
+class _DummyWriter(docutils.writers.Writer):  # type: ignore[type-arg]
+    # compat for MyST-Parser
+    supported = ('html',)
+
+
 if docutils.__version_info__[:2] < (0, 22):
     from docutils.parsers.rst import roles