Fix exception type in test_retrieval (#7)

Using json.JSONDecodeError should replace using the 'message'
accessor of exceptions, which Python 3 removed.
This works as long as "simplejson" is not added as project
dependency, because "requests" gives preferential treatment to
simplejson, which ships its own version of JSONDecodeError.

tests/test_retrieval.py

@@ -1,5 +1,6 @@
 # -*- encoding: utf-8 -*-
 
+import json
 import logging
 import os
 import pytest
@@ -45,9 +46,8 @@ def test_retrieval(canteen):
 
     try:
         menu = download_menu(params)
-    except ValueError as e:
+    except json.JSONDecodeError as e:
-        if e.message == 'No JSON object could be decoded':
         pytest.xfail('JSON endpoint returned garbage (issue #6)')
-        raise e
+        raise e  # Appease PyCharm inspection - xfail always raises
 
     feed.render_menu(menu)