Bring back type annotation support of dunder methods in stub generator (#12828)

Fixes #12717

Author: bluenote10

mypy/stubgenc.py

@@ -288,7 +288,7 @@ def infer_prop_type(docstr: Optional[str]) -> Optional[str]:
             return None
 
     # Ignore special properties/attributes.
-    if name.startswith('__') and name.endswith('__'):
+    if is_skipped_attribute(name):
         return
 
     inferred = infer_prop_type(getattr(obj, '__doc__', None))

test-data/stubgen/pybind11_mypy_demo/basics.pyi

@@ -5,6 +5,7 @@ PI: float
 
 class Point:
     class AngleUnit:
+        __members__: ClassVar[dict] = ...  # read-only
         __entries: ClassVar[dict] = ...
         degree: ClassVar[Point.AngleUnit] = ...
         radian: ClassVar[Point.AngleUnit] = ...
@@ -20,6 +21,7 @@ class Point:
         def name(self) -> str: ...
 
     class LengthUnit:
+        __members__: ClassVar[dict] = ...  # read-only
         __entries: ClassVar[dict] = ...
         inch: ClassVar[Point.LengthUnit] = ...
         mm: ClassVar[Point.LengthUnit] = ...