Fix #8041: autodoc: An ivar on super class is not shown unexpectedly

An annotated instance variable on super class is not documented when
derived class has also other annotated instance variables because
`obj.__annotations__` is overrided by derived class's type hints.

To get annotations of the target class correctly, this scans MRO to
get all of annotated instance variables.

CHANGES

@@ -63,6 +63,8 @@ Bugs fixed
   when ``:inherited-members:`` option given
 * #8032: autodoc: A type hint for the instance variable defined at parent class
   is not shown in the document of the derived class
+* #8041: autodoc: An annotated instance variable on super class is not
+  documented when derived class has other annotated instance variables
 * #7839: autosummary: cannot handle umlauts in function names
 * #7865: autosummary: Failed to extract summary line when abbreviations found
 * #7866: autosummary: Failed to extract correct summary line when docstring