PY3 Migration- make keras/callbacks.py py3 compatible.

PiperOrigin-RevId: 280509492
Change-Id: I4ba0f66b13533f3169fd61bfcf84d7d735378ff5

tensorflow/python/keras/BUILD

@@ -1423,8 +1423,10 @@ tf_py_test(
         ":keras",
         "@absl_py//absl/testing:parameterized",
         "//third_party/py/numpy",
+        "@six_archive//:six",
         "//tensorflow/python:client_testlib",
     ],
+    python_version = "PY3",
     shard_count = 4,
     tags = [
         "no_oss",

tensorflow/python/keras/callbacks.py

@@ -1041,8 +1041,8 @@ class ModelCheckpoint(Callback):
 
         self._maybe_remove_file()
       except IOError as e:
-        # `e.errno` appears to be `None` so checking the content of `e.message`.
-        if 'is a directory' in e.message:
+        # `e.errno` appears to be `None` so checking the content of `e.args[0]`.
+        if 'is a directory' in six.ensure_str(e.args[0]):
           raise IOError('Please specify a non-directory filepath for '
                         'ModelCheckpoint. Filepath used is an existing '
                         'directory: {}'.format(filepath))