misc: don't recurse in attachments proxy when deepcopying (#36515)

wcs/workflows.py

@@ -155,6 +155,8 @@ class AttachmentsSubstitutionProxy(object):
         self.formdata = formdata
 
     def __getattr__(self, name):
+        if name.startswith('__'):
+            raise AttributeError(name)
         for evo in self.formdata.evolution or []:
             for part in evo.parts or []:
                 if getattr(part, 'varname', None) == name: