Fix tests in python2

slapcache/test/test_signature.py

 from slapcache.signature import Signature, Config, strategy
 from slapos.libnetworkcache import NetworkcacheClient
-from optparse import Values
-import slapos.signature, time, difflib, tempfile, unittest, os
+import slapos.signature, tempfile, unittest, os, sys
 def _fake_call(self, *args, **kw):
   self.last_call = (args, kw)
 
@@ -146,7 +145,10 @@ signature-certificate-list =
 
     def test_configuration_file_dont_exist(self):
         self.config_dict.slapos_configuration = '/abc/123'
-        self.assertRaises(FileNotFoundError, Signature, self.config_dict)
+        if sys.version_info.major < 3:
+            self.assertRaises(IOError, Signature, self.config_dict)
+        else:
+            self.assertRaises(FileNotFoundError, Signature, self.config_dict)
 
     def test_download_not_existing(self):
         _, path = tempfile.mkstemp()