Flaky test: Oauth2Test.test_base64

https://github.com/apache/pulsar-client-python/actions/runs/5493895300/jobs/10012373710?pr=138

```
======================================================================
2023-07-08 10:09:29.109 ERROR [139724761126464] ClientImpl:214 | Error Checking/Getting Partition Metadata while creating producer on persistent://public/default/oauth2-test-base64 -- AuthenticationError
ERROR: test_base64 (__main__.Oauth2Test)
2023-07-08 10:09:29.110 INFO [139724795805696] ClientImpl:614 | Closing Pulsar client with 0 producers and 0 consumers
----------------------------------------------------------------------
2023-07-08 10:09:29.110 INFO [139724761126464] ClientConnection:263 | [[::1]:45552 -> [::1]:6650] Destroyed connection
Traceback (most recent call last):
File "/home/runner/work/pulsar-client-python/pulsar-client-python/tests/oauth2_test.py", line 69, in test_base64
producer = client.create_producer('oauth2-test-base64')
File "/home/runner/.local/lib/python3.10/site-packages/pulsar/__init__.py", line 684, in create_producer
p._producer = self._client.create_producer(topic, conf)
_pulsar.TopicNotFound: Pulsar error: TopicNotFound

======================================================================
ERROR: test_key_file (__main__.Oauth2Test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/runner/work/pulsar-client-python/pulsar-client-python/tests/oauth2_test.py", line 53, in test_key_file
producer = client.create_producer('oauth2-test-base64')
File "/home/runner/.local/lib/python3.10/site-packages/pulsar/__init__.py", line 684, in create_producer
p._producer = self._client.create_producer(topic, conf)
_pulsar.TopicNotFound: Pulsar error: TopicNotFound
```

從 tests/oauth2_test.py 開始，尤其查看 Oauth2Test.test_base64 和 test_key_file，接著檢查連結的 GitHub Actions 執行記錄以尋找 TopicNotFound 失敗。確定 OAuth2 測試在建立 producer 時間歇性失敗的原因；當這些測試能在 CI 中可靠地通過時，即表示完成。

由索引模型根據 Issue 內容生成。