Fix cp.push test

mock_open now properly handles bytestrings, so the expected data needed
to be updated.

tests/unit/modules/test_cp.py

@@ -147,7 +147,7 @@ class CpTestCase(TestCase, LoaderModuleMockMixin):
                     cmd='_file_recv',
                     tok='token',
                     path=['saltines', 'test.file'],
-                    data='',  # data is empty here because load['data'] is overwritten
+                    data=b'',  # data is empty here because load['data'] is overwritten
                     id='abc'
                 )
             )