Sayt

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**Bugfixes**

- Fix a bug that different Python system use different pickle protocol for cache key.

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**Miscellaneous**

- Add support for Python3.7 for backward compatibility.

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**Bugfixes**

- Fix a bug caused by the logger.

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**Features and Improvements**

- Be able to serialize and deserialize ``BaseField`` object.

**Miscellaneous**

- Add downloading block to logging message.

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**💥 Breaking change**

- Rework the ``DataSet`` class, merge features in ``RefreshableDataSet`` into the ``DataSet`` class.
- ``RefreshableDataSet`` is removed.

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**Bugfixes**

- Fix a bug that the ``query`` parameter in ``RefreshableDataSet.search`` should be mandatory.