fix behavior of get_attributes()

* make sure it returns an ordered list in both python 2 and python 3

ldapcherry/attributes.py

@@ -130,7 +130,9 @@ class Attributes:
     def get_backend_attributes(self, backend):
         if backend not in self.backends:
             raise WrongBackend(backend)
-        return self.backend_attributes[backend].keys()
+        ret = list(self.backend_attributes[backend].keys())
+        ret.sort()
+        return ret
 
     def get_backend_key(self, backend):
         if backend not in self.backends:

tests/test_Attributes.py

@@ -42,6 +42,7 @@ class TestError(object):
         inv = Attributes('./tests/cfg/attributes.yml')
         ret = inv.get_backend_attributes('ldap')
         expected = ['shell', 'cn', 'userPassword', 'uidNumber', 'gidNumber', 'sn', 'home', 'givenName', 'email', 'uid']
+        expected.sort()
         assert ret == expected
 
     def testGetKey(self):