Remove unnecessary calls to dict.keys()

iter(dict) is identical to iter(dict.keys()). The call to .keys() is
unnecessary and only adds noise. Remove these unnecessary calls.

Inspired by Lennart Regebro's presentation "Prehistoric Patterns' at
PyCon 2017. Available at:

https://www.youtube.com/watch?v=V5-JH23Vk0I

import_export/fields.py

@@ -62,8 +62,7 @@ class Field(object):
             value = data[self.column_name]
         except KeyError:
             raise KeyError("Column '%s' not found in dataset. Available "
-                           "columns are: %s" % (self.column_name,
-                                                list(data.keys())))
+                           "columns are: %s" % (self.column_name, list(data)))
 
         try:
             value = self.widget.clean(value, row=data)

import_export/resources.py

@@ -586,7 +586,7 @@ class Resource(six.with_metaclass(DeclarativeMetaclass)):
 
     def get_export_order(self):
         order = tuple(self._meta.export_order or ())
-        return order + tuple(k for k in self.fields.keys() if k not in order)
+        return order + tuple(k for k in self.fields if k not in order)
 
     def before_export(self, queryset, *args, **kwargs):
         """

tests/core/tests/instance_loaders_tests.py

@@ -25,7 +25,7 @@ class CachedInstanceLoaderTest(TestCase):
     def test_all_instances(self):
         self.assertTrue(self.instance_loader.all_instances)
         self.assertEqual(len(self.instance_loader.all_instances), 1)
-        self.assertEqual(list(self.instance_loader.all_instances.keys()),
+        self.assertEqual(list(self.instance_loader.all_instances),
                          [self.book.pk])
 
     def test_get_instance(self):