make test_var_name compatible with nosetests3

tests/test_var_names.py

@@ -17,7 +17,7 @@ def test_var_names():
             data = file.read().replace('\n', '')    
 
         lc_vars = [x.lower() for x in variable_re.findall(data)]
-        doc = lxml.etree.fromstring(data)
+        doc = lxml.etree.parse(file_name)
         pkg_vars = doc.xpath('//variable')
         allowed_pkg_vars = [x.get('name') for x in pkg_vars]