Lookup instance attibutes on obj in MRO

Instance attirbutes on objects defined in other modules cannot be
found by module analyzer unless we import that module. So try
looking for instance attirbutes on objects in MRO order

sphinx/ext/autosummary/__init__.py

@@ -81,7 +81,7 @@
 from sphinx.util import logging, rst
 from sphinx.util.docutils import (NullReporter, SphinxDirective, SphinxRole, new_document,
                                   switch_source_input)
-from sphinx.util.inspect import signature_from_str
+from sphinx.util.inspect import getmro, signature_from_str
 from sphinx.util.matching import Matcher
 from sphinx.util.typing import OptionSpec
 from sphinx.writers.html import HTMLTranslator
@@ -711,15 +711,20 @@ def import_ivar_by_name(name: str, prefixes: List[str] = [None],
     try:
         name, attr = name.rsplit(".", 1)
         real_name, obj, parent, modname = import_by_name(name, prefixes, grouped_exception)
-        qualname = real_name.replace(modname + ".", "")
-        analyzer = ModuleAnalyzer.for_module(getattr(obj, '__module__', modname))
-        analyzer.analyze()
-        # check for presence in `annotations` to include dataclass attributes
-        attrs_found = {name_attr[1] for name_attr in analyzer.attr_docs} | {
-            name_attr[1] for name_attr in analyzer.annotations
-        }
-        if attr in attrs_found:
-            return real_name + "." + attr, INSTANCEATTR, obj, modname
+
+        candidate_objects = getmro(obj)
+        if len(candidate_objects) == 0:
+            candidate_objects = (obj,)
+
+        for candidate_obj in candidate_objects:
+            analyzer = ModuleAnalyzer.for_module(getattr(candidate_obj, '__module__', modname))
+            analyzer.analyze()
+            # check for presence in `annotations` to include dataclass attributes
+            attrs_found = {name_attr[1] for name_attr in analyzer.attr_docs} | {
+                name_attr[1] for name_attr in analyzer.annotations
+            }
+            if attr in attrs_found:
+                return real_name + "." + attr, INSTANCEATTR, obj, modname
     except (ImportError, ValueError, PycodeError) as exc:
         raise ImportError from exc
     except ImportExceptionGroup: