Update openapi-generator to fix urllib3 deprecation warning

[quantus]

After updating urllib3 to more recent version (due to CVE-2023-43804) I'm seeing following warning:

[REDACTED].py: in generate_pdf
    response = doc_api.create_async_doc(options)
../venv/lib/python3.10/site-packages/docraptor/api/doc_api.py:67: in create_async_doc
    return self.create_async_doc_with_http_info(doc, **kwargs)  # noqa: E501
../venv/lib/python3.10/site-packages/docraptor/api/doc_api.py:172: in create_async_doc_with_http_info
    return self.api_client.call_api(
../venv/lib/python3.10/site-packages/docraptor/api_client.py:378: in call_api
    return self.__call_api(resource_path, method,
../venv/lib/python3.10/site-packages/docraptor/api_client.py:185: in __call_api
    response_data = self.request(
../venv/lib/python3.10/site-packages/docraptor/api_client.py:422: in request
    return self.rest_client.POST(url,
../venv/lib/python3.10/site-packages/docraptor/rest.py:272: in POST
    return self.request("POST", url,
../venv/lib/python3.10/site-packages/docraptor/rest.py:231: in request
    raise ApiException(http_resp=r)
../venv/lib/python3.10/site-packages/docraptor/exceptions.py:110: in __init__
    self.headers = http_resp.getheaders()
../venv/lib/python3.10/site-packages/docraptor/rest.py:42: in getheaders
    return self.urllib3_response.getheaders()
../venv/lib/python3.10/site-packages/urllib3/response.py:667: in getheaders
    warnings.warn(
E   DeprecationWarning: HTTPResponse.getheaders() is deprecated and will be removed in urllib3 v2.1.0. Instead access HTTPResponse.headers directly.

This issue has been fixed in this PR of openapi-generator and that the fix was released on version v6.4.0, but this repository is using an older version:

docraptor-python/.generator-revision

Line 1 in b8b08c1

v6.0.1