Fix Stubgen's behavior for Instance Variables in C extensions

It is not necessary for instance variables to have the fget attrbute
(e.g. instance variable in a C class in an extension) but
inspect.isdatadescriptor return True as expected, hence we update
the 'is_c_property' check.

Since special attributes (like __dict__ etc) also passes 'is_c_property'
check, we ignore all such special attributes in
'generate_c_property_stub' while creating the contents of stub file.

Also, 'is_c_property_readonly' assumed that the property would always
have 'fset' attribute which again is not true for instance variables
in C extension. Hence make the check defensive by first checking if
'fset' attribute even exists or not.

This fixes #12150.

Author: sishubha

mypy/stubgenc.py

@@ -121,11 +121,11 @@ def is_c_classmethod(obj: object) -> bool:
 
 
 def is_c_property(obj: object) -> bool:
-    return inspect.isdatadescriptor(obj) and hasattr(obj, 'fget')
+    return inspect.isdatadescriptor(obj) or hasattr(obj, 'fget')
 
 
 def is_c_property_readonly(prop: Any) -> bool:
-    return prop.fset is None
+    return hasattr(prop, 'fset') and prop.fset is None
 
 
 def is_c_type(obj: object) -> bool:
@@ -287,6 +287,10 @@ def infer_prop_type(docstr: Optional[str]) -> Optional[str]:
         else:
             return None
 
+    # Ignore special properties/attributes.
+    if name.startswith('__') and name.endswith('__'):
+        return
+
     inferred = infer_prop_type(getattr(obj, '__doc__', None))
     if not inferred:
         fget = getattr(obj, 'fget', None)