Toloka-kit

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* Downgraded attrs dependency from `attrs>=21.2.0` to `attrs>=20.3.0`. See https://github.com/Toloka/toloka-kit/issues/37
* Cursor states are now updated after all callbecks are run successfully (toloka.straming)
* String values passed for Enum-annotated arguments are now automatically converted to Enums

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* `toloka.client` objects are not serializable
* Metrics are introduced

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* Streaming library to build pipelines
* Examples refactoring

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* Added new template builder components
* Added the ability to translate project title and description into multiple languages
* ExtendableStrEnum class has been added for forward compatibility with new enumeration keys

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* Added new `TemplateBuilder` components
* Deleted `name` from `UserSkillSearchRequest`
* Passing `Retry` object into `TolokaClient.retries` is now deprecated. Use new `TolokaClient.retryer_factory` argument instead.
* Added `__all__` to stub files
* Fixed an exception raising when opening an open pool
* Fixed `TemplateBuilder` view-components bug with the disappearance of the `hint`, `label` and `validation` fields in `0.1.10` version.

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* `TemplateBuilder` components now support positional arguments
* Added `readonly` flag to attributes
* Added `ACCEPT_AFTER_REJECT` value to ` AssessmentEvent.Type`
* Added `assignment_id` attribute to `UserBonusSearchRequest`
* Improved stub files formatting