Clean-python

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- Fixed `ImportError` because of absent `asyncpg` when using only `sql-sync` dependency.

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- Added a base class for providers (`Provider` / `SyncProvider`) with empty
definitions of `connect` and `disconnect` (co)routines.

- Re-use the `S3Client` in `S3Provider`. Awaiting `S3Provider.connect()` at
startup is now necessary.

- Re-use the `ClientSession` in `ApiProvider`. Awaiting `ApiProvider.connect()`
at startup is now necessary.

- Added `SyncInternalGateway`.

- Breaking change in `InternalGateway`: it now requires a `.mapper` attribute
instead of `.to_internal` and `.to_external` methods.

- Adapted underscore to hyphen in "extra" requirements: `sql_sync -> sql-sync`,
`api_client -> api-client`. See PEP685.

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- Added synchronous S3 interface (installable through optional `s3_sync` dependency).

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- Added YAML version of the openapi spec under my.domain/v1/openapi.yaml.

- Fixed an issue introduced in 0.12.4 with the securitySchemas in the openapi spec. Now there
is always 1 securitySchema named "OAuth2.

- Replace the `auth` setting in the `fastapi.Service` with a different type, which encompasses
the (original) token verifier settings, oauth2 settings, and a new scope_verifier callable.

- Scopes supplied per-endpoint are now documented in the OpenAPI schema.

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- Allow self-signed certificates in CeleryRmqBroker.

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- Fixed synchronous usage of DomainEvent.

- Breaking change: the allowed values in `RequestQuery.order_by` should now be
specified using a literal type (for example `Literal["id", "-id"]`) instead of the
`enum` keyword argument. When using the `enum` keyword argument, an exception will be
raised. Upside of this is that the OpenAPI spec now correctly lists the options for `order_by`.

- Exceptions raised in validators of `RequestQuery` subclasses now result in a `BadRequest`
instead of an internal server error. This requires change: instances of `Depends(SomeQuery)`
must be replaced by `SomeQuery.depends()`.