Piccolo-api

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When using the ``swagger_ui`` endpoint, the title can now be customised -
courtesy heliumbrain.

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* Added an ``allow_unauthenticated`` option to ``SessionsAuthBackend``, which
will add an ``UnauthenticatedUser`` to the scope, instead of rejecting the
request. The app's endpoints are then responsible for checking
``request.user.is_authenticated``.
* Improved the docs for Session Auth.
* If ``deserialize_json`` is False on ``create_pydantic_model``, it will
still provide some JSON validation.

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Added a ``deserialize_json`` option to ``create_pydantic_model``, which will
convert JSON strings to objects - courtesy heliumbrain.

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Added the OAuth redirect endpoint to ``swagger_ui``.

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Added a ``swagger_ui`` endpoint which works with Piccolo's ``CSRFMiddleware``.

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Modified the auth middleware to add the Piccolo ``BaseUser`` instance for the
authenticated user to Starlette's ``BaseUser``.

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