Can't serialize graphs using Retry

[fitzoh]

Just installed v1.1.2

from stepfunctions.steps import Graph, Task, states, Retry
retry = Retry(error_equals=["States.TaskFailed"])
Task(state_id="whatever", retry=retry).to_json()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/fitz/Workspace/github.com/mobikitinc/data-ingest/venv/lib/python3.8/site-packages/stepfunctions/steps/states.py", line 93, in to_json
    return json.dumps(self.to_dict())
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type Retry is not JSON serializable

[ca-nguyen]

A fix was merged in #169

This will be included in the next release