Loosened backend constraints

[RikvanToor]

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This PR changes the creation of SqlBackends to some type backend for which the constraints IsPersistBackend backend, BaseBackend backend ~ SqlBackend, BackendCompatible SqlBackend backend hold. This allows the user to use this package with other backends than just SqlBackend. My own use case for this is to be able to use SqlReadBackend. This way, I can run read-only queries on a different connection than writing queries.