Fix UnionType.__setattr__ in Shadow.py (GH-4727)

The condition should only evaluate to True when assigning __dict__, but it currently does for _, d, i, etc. as well as resulting in the following potential issues:

* Non-member are being assigned to the object instead of raising.
* The one-field rule can be bypassed.
* Valid members that pass the condition are being assigned raw and are never cast to the specified type.

Cython/Shadow.py

@@ -305,7 +305,7 @@ class UnionType(CythonType):
             setattr(self, key, value)
 
     def __setattr__(self, key, value):
-        if key in '__dict__':
+        if key == '__dict__':
             CythonType.__setattr__(self, key, value)
         elif key in self._members:
             self.__dict__ = {key: cast(self._members[key], value)}