Quart-db

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* Bugfix ensure the value of data_loaded is kept.

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* Bugfix naming typo.
* Bugfix background migrations should not run in a transaction.

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* Make the valid_migration function optional, if not present the
migration is assumed to be valid.
* Support background migrations. Note the first deployment should be
monitored as this requires a change to Quart-DB's state table.
* Bugfix ensure None is returned if there is no result for
``fetch_val`` and ``fetch_one``.

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* Ensure auto request connectons are released.

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* Add a migrations timeout defaulting to 60s.
* Support Quart 0.19 onwards.
* Support 3.12 drop 3.7.

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* Bugfix ensure connections are released.