gyp: list(dict) so we can del dict(key) while iterating

Fixes #1998

Reviewed-By: Rod Vagg <rod@vagg.org>
PR-URL: #2009

Author: cclauss

gyp/pylib/gyp/input.py

@@ -2286,7 +2286,7 @@ def SetUpConfigurations(target, target_dict):
         merged_configurations[configuration])
 
   # Now drop all the abstract ones.
-  for configuration in target_dict['configurations'].keys():
+  for configuration in list(target_dict['configurations']):
     old_configuration_dict = target_dict['configurations'][configuration]
     if old_configuration_dict.get('abstract'):
       del target_dict['configurations'][configuration]