[Python] Bug when working with FakeDatetime from freezegun

Swagger code generation for datetime fails in the context of fake datetime substituted by freezgun.

Description

Swagger-codegen version

3.0.20

Swagger declaration file content or url

https://api.cifrum.io/assets/swagger.json

Command line used for generation

$ java -jar ~/apps/swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
   -i https://api.cifrum.io/assets/swagger.json \
   -l python \
   -o ./lib/api_swagger_client/

Steps to reproduce

1. Create a test inside the context of freezgun
2. Try to fetch a model with a datetime field
3. It should fail with the stack trace ending with:

  File "/lib-python/lib/api_swagger_client/swagger_client/api/infos_api.py", line 126, in mutru_info_with_http_info
    collection_formats=collection_formats)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 321, in call_api
    _preload_content, _request_timeout)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 160, in __call_api
    return_data = self.deserialize(response_data, response_type)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 232, in deserialize
    return self.__deserialize(data, response_type)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 271, in __deserialize
    return self.__deserialize_model(data, klass)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 614, in __deserialize_model
    kwargs[attr] = self.__deserialize(value, attr_type)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 271, in __deserialize
    return self.__deserialize_model(data, klass)
  File "/lib-python/lib/api_swagger_client/swagger_client/api_client.py", line 604, in __deserialize_model
    if not klass.swagger_types and not self.__hasattr(klass, 'get_real_child_model'):
AttributeError: type object 'datetime.datetime' has no attribute 'swagger_types'

It fails because api_client.py:268 check klass == datetime.datetime fails since datetime.datetime is actually <class 'freezegun.api.FakeDatetime'>.

Suggest a fix/enhancement

Hopefully FakeDatetime overrides issubclass. And I suggest the fix as follows:

        elif issubclass(klass, datetime.date):
            return self.__deserialize_date(data)
        elif issubclass(klass, datetime.datetime):
            return self.__deserialize_datatime(data)

Start in the generated Python client's api_client.py around line 268 and follow the __deserialize and __deserialize_model calls shown in the stack trace. Reproduce the failure with freezegun and a model containing a datetime field; done means FakeDatetime values deserialize without the swagger_types AttributeError.