ISSUE-6400: Cpp python test failed

[sijie]

Original Issue: apache#6400

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Describe the bug
Python test failed:

You should consider upgrading via the 'pip install --upgrade pip' command.
---- Running Python unit tests
/tmp /pulsar/pulsar-client-cpp/python /pulsar/pulsar-client-cpp
Traceback (most recent call last):
  File "pulsar_test.py", line 26, in <module>
    from pulsar import Client, MessageId, \
  File "/usr/local/lib/python2.7/dist-packages/pulsar/__init__.py", line 109, in <module>
    from pulsar import schema
  File "/usr/local/lib/python2.7/dist-packages/pulsar/schema/__init__.py", line 21, in <module>
    from .definition import *
  File "/usr/local/lib/python2.7/dist-packages/pulsar/schema/definition.py", line 21, in <module>
    from enum import Enum, EnumMeta
ImportError: No module named enum
##[error]Process completed with exit code 1.

To Reproduce
https://github.com/apache/pulsar/pull/6392/checks?check_run_id=461910053

Expected behavior
Test pass

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• Github action

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