se implementa un esquema para modelar el xml

FossilOrigin-Name: e4de658f60fe8fcbb330923e14958a5d8f8e0e6395db4f992ec7da45062fa193

facho/model/fields/attribute.py

@@ -0,0 +1,17 @@
+from .field import Field
+
+class Attribute(Field):
+    def __init__(self, tag):
+        self.tag = tag
+  
+    def __get__(self, inst, cls):
+        if inst is None:
+            return self
+
+        assert self.name is not None
+        (tag, value) =  inst._xml_attributes[self.name]
+        return value
+
+    def __set__(self, inst, value):
+        assert self.name is not None
+        inst._xml_attributes[self.name] = (self.tag, value)