Handle incomplete response

[cliffckerr]

If the output is too long and gets truncated, then

        json = self.response.to_json()
        result_string = json['kwargs']['content']
        match_pattern = re.compile(r'```python(.*?)```', re.DOTALL)
        code_match = match_pattern.search(result_string)
        self.new_str = code_match.group(1)

fails with

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/cliffk/idm/starsim_ai/starsim_ai/migration.py", line 80, in parse_response
    self.new_str = code_match.group(1)
                   ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'group'

Need to make more robust -- only search for ```python, and then separately search for ``` at the end, perhaps?