Use future.utils iterkeys; fix some more style issues

python/ycm/diagnostic_filter.py

@@ -23,6 +23,7 @@ from future import standard_library
 standard_library.install_aliases()
 from builtins import *  # noqa
 
+from future.utils import iterkeys
 import re
 
 
@@ -30,7 +31,7 @@ class DiagnosticFilter( object ):
   def __init__( self, config ):
     self._filters = []
 
-    for filter_type in config.iterkeys():
+    for filter_type in iterkeys( config ):
       wrapper = _AsIs
       actual_filter_type = filter_type