list.copy() is not compatible with python 2.7

I doubt that a .copy() is required there since list() would already create one but since the previous committer added it I am improving by replacing the .copy() with a [:] which makes a copy in python 2.7 and 3+

salt/cloud/__init__.py

@@ -240,7 +240,7 @@ class CloudClient(object):
                          if a.get('provider', '')]
             if providers:
                 _providers = opts.get('providers', {})
-                for provider in list(_providers).copy():
+                for provider in list(_providers)[:]:
                     if provider not in providers:
                         _providers.pop(provider)
         return opts