Quetzal-client

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* Update to API version 0.5.0.
* Add API key authentication.

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* Update to API version 0.3.0.
* Add more helper functions (query and client).
* Fix incorrect call to derived configuration object.
* Fix unpickeable exception types.
* Fix bugs on file download checksum verification and file upload parameters.

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* Update dependency *quetzal-openapi-client* to match Quetzal API version 0.2.0.
* Fix ``dependency_links`` to correctly link to *quetzal-openapi-client*.

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* ``quetzal.client.api`` has been renamed to ``quetzal.client.helpers``.
* File download, with verification of known files, added to helpers.
* File download falls back to user data directory if output not set.
* Config default to environment variable values when created.
* Initial structure of Sphinx documentation.

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* First iteration of the Quetzal Python client.
* Delegates the *automatic* code to `quetzal-openapi-client <https://github.com/quetz-al/quetzal-openapi-client>`_.
* Command-line interface implemented with `Click <https://palletsprojects.com/p/click/>`_
for most operations available on the Quetzal API.
* Incomplete implementation of a helper module `quetzal.client.api` that
encapsulates the usage of the auto-generated client.