Fix #8534: autoattribute failed to document a commented attribute in alias class

So far, autoattribute uses "given class name" to fetch comments of
uninitialized instance attributes.  But pycode expects to use "real"
class name on searching attribute comments.

This adds UninitializedInstanceAttributeMixin to handle it simply.

tests/roots/test-ext-autodoc/target/typed_vars.py

@@ -29,3 +29,6 @@ class Class:
 
 class Derived(Class):
     attr7: int
+
+
+Alias = Class