Raise exception where original is not in the __dict__ of an object

This prevents problems where attempts are made to replace an attribute on a subclass instead of the base class.

testfixtures/replace.py

@@ -109,9 +109,23 @@ def __call__(
 
         if resolved.setter is None:
             raise ValueError('target must contain at least one dot!')
+
         if resolved.found is not_there and strict:
             raise AttributeError('Original %r not found' % resolved.name)
 
+        if (
+                hasattr(resolved.container, '__dict__') and
+                resolved.setter is setattr and
+                resolved.name not in resolved.container.__dict__
+        ):
+            if strict:
+                raise AttributeError(
+                    f'{resolved.container!r} has __dict__ but {resolved.name!r} is not in it'
+                )
+            else:
+                resolved.found = not_there
+
+
         replacement_to_use = replacement
 
         if isinstance(resolved.container, type):