fix: training loop example for sumo rl

[k223kim]

I was getting this error when running the training script provided in README.md and running sumo_rl_simple.py:

Traceback (most recent call last):
  File "/teamspace/studios/this_studio/OpenEnv/src/main.py", line 18, in <module>
    result = env.step(SumoAction(phase_id=action_id))
  File "/teamspace/studios/this_studio/OpenEnv/src/core/http_env_client.py", line 136, in step
    r = self._http.post(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/sessions.py", line 637, in post
    return self.request("POST", url, data=data, json=json, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/sessions.py", line 575, in request
    prep = self.prepare_request(req)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/sessions.py", line 484, in prepare_request
    p.prepare(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/models.py", line 370, in prepare
    self.prepare_body(data, files, json)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/models.py", line 510, in prepare_body
    body = complexjson.dumps(json, allow_nan=False)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type int64 is not JSON serializable

This PR fixes the error so that the example can run successfully.

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