fix: util.Namespace follows Attribute protocol so inspect/pytest/et al work again (#270)

I make use of `signxml` for SAML XML verification.

I recently tried to make `pytest` validate all my doctests, however due
to `util.Namespace` not following the Attribute protocol (i.e. an
override of `__getattr__`) -- on missing attributes, one should
**always** raise `AttributeError` as that allows any introspection via
the `inspect` module to "Just Work".

I was able to diagnose that the error was identical to
pytest-dev/pytest#5080 and that a remediation
was simply catching `KeyError` and re-raising as `AttributeError`.

Author: autumnjolitz

signxml/util/__init__.py

@@ -25,7 +25,10 @@
 
 class Namespace(dict):
     def __getattr__(self, a):
-        return dict.__getitem__(self, a)
+        try:
+            return dict.__getitem__(self, a)
+        except KeyError:
+            raise AttributeError(a) from None
 
 
 namespaces = Namespace(