👌 IMPROVE: parse_directive_options

The change allows compatibility with the
`sphinx.ext.autodoc.directive.DummyOptionSpec` instances.

myst_parser/parse_directives.py

@@ -142,8 +142,9 @@ def parse_directive_options(
     # check options against spec
     options_spec = directive_class.option_spec  # type: Dict[str, Callable]
     for name, value in list(options.items()):
-        convertor = options_spec.get(name, None)
-        if convertor is None:
+        try:
+            convertor = options_spec[name]
+        except KeyError:
             raise DirectiveParsingError(f"Unknown option: {name}")
         if not isinstance(value, str):
             if value is True or value is None: