[python][client] allow passing floats for _request_timeout

[daniel-k]

While several method docstrings suggested that you could pass floats for _request_timeout and the underlying urllib3.Timeout can deal with them, the code only allowed integer values so far when passed as a single number.

>>> # everything alright
>>> api.my_endpoint(_request_timeout=2)
>>>
>>> api.my_endpoint(_request_timeout=1.2)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File ".../client/api_client.py", line 763, in __call__
    return self.callable(self, *args, **kwargs)
  File ".../client/api/my_api.py", line 344, in __list_appliances
    return self.call_with_http_info(**kwargs)
  File ".../client/api_client.py", line 810, in call_with_http_info
    self.__validate_inputs(kwargs)
  File ".../client/api_client.py", line 704, in __validate_inputs
    fixed_val = validate_and_convert_types(
  File ".../client/model_utils.py", line 1394, in validate_and_convert_types
    converted_instance = attempt_convert_item(
  File ".../client/model_utils.py", line 1269, in attempt_convert_item
    valid_classes_ordered = order_response_types(valid_classes)
  File ".../client/model_utils.py", line 900, in order_response_types
    sorted_types = sorted(
  File ".../client/model_utils.py", line 902, in <lambda>
    key=lambda class_or_instance: index_getter(class_or_instance)
  File ".../client/model_utils.py", line 898, in index_getter
    raise ApiValueError("Unsupported type: %s" % class_or_instance)
my_client.exceptions.ApiValueError: Unsupported type: <class 'tuple'>

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CC @taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess @arun-nalla @spacether

[wing328]

Travis CI failure not related to this change. CircleCI tests already fixed in the master.