Versioned

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**Features and Improvements**

- Add ``Repository.list_artifact_names()`` and ``Repository.purge_artifact_versions()`` methods to both ``s3_and_dynamodb_backend`` and ``s3_only_backend``

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**Features and Improvements**

- Fix a bug that the ``s3_only_backend`` is not able to handle suffix like ``.tar.gz``, ``.json.gz``.

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**Minor Improvements**

- add validation to detect if human manually contaminate the artifact S3 dir.
- add the missing ``Alias.random_artifact()`` method.

**Bugfixes**

- fix bug that sometimes it uses the wrong boto session to get the S3 metadata and S3 last modified time.

**Miscellaneous**

- Update the s3 and dynamodb backend example jupyter notebook.
- Add s3 only backend example jupyter notebook.

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**Bugfixes**

- fix a bug that accidentally deleted the ``get_artifact_s3path`` API.

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**Features and Improvements**

- add S3 only backend in ``s3_only_backend.py`` module. It doesn't require DynamoDB table.
- the old ``core.py`` module is renamed to ``s3_and_dynamodb_backend.py`` module. The old API is kept intentionally for backward compatibility. However, it is scheduled to be removed in 1.X.Y release.
- suggest to use ``from versioned import api as versioned``, then use ``versioned.s3_and_dynamodb_backend`` or ``versioned.s3_only_backend`` to access the backend.

**Minor Improvements**

- improve test for ``s3_and_dynamodb_backend``.

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**Bugfixes**

- fix a bug that the ``purge_all`` method cannot delete DynamoDB items correctly when S3 doesn't have the artifacts.