Fix Failing test for JSON Formatter on Python 3.8

[kaxil]

This test is failing on Master:

tests/utils/log/test_es_task_handler.py ..................F

_ TestElasticsearchTaskHandler.test_set_context_w_json_format_and_write_stdout _

self = <tests.utils.log.test_es_task_handler.TestElasticsearchTaskHandler testMethod=test_set_context_w_json_format_and_write_stdout>

    def test_set_context_w_json_format_and_write_stdout(self):
        self.es_task_handler.formatter = mock.MagicMock()
        self.es_task_handler.formatter._fmt = mock.MagicMock()
        self.es_task_handler.formatter._fmt.find = mock.MagicMock(return_value=1)
        self.es_task_handler.write_stdout = True
        self.es_task_handler.json_format = True
>       self.es_task_handler.set_context(self.ti)

tests/utils/log/test_es_task_handler.py:262: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
airflow/utils/log/es_task_handler.py:210: in set_context
    self.formatter = JSONFormatter(
airflow/utils/log/json_formatter.py:35: in __init__
    super().__init__(fmt, datefmt, style)
/usr/local/lib/python3.8/logging/__init__.py:576: in __init__
    self._style.validate()

Also, there is no need of passing the default fmt value explicitly as it is the default.

cc @andriisoldatenko

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[kaxil]

@andriisoldatenko Can you let me know if I am missing something here please