Clean-python

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- Fixed celery task_failure_log in case of a crashed worker.

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- SyncApiProvider: also retry when the Retry-After response header is missing.

- ApiProvider: (sync and async) retry on all methods except POST.

- ApiProvider: (sync and async) retry on 429, 500, 502, 503, 504.

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- Use a timeout for fetching jwks in TokenVerifier.

- Changed celery task logger args/kwargs back to argsrepr/kwargsrepr.

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- Sanitize error responses.

- Remove 501 error response on NotImplementedError.

- Solved aiohttp 'Unclosed client session' warning.

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- Revert changes done in 0.9.1 in CCTokenGateway.

- Added CCTokenGateway.fetch_headers()

- Added optional 'headers' parameter to ApiProvider.

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- Renamed 'fetch_token' parameter in api client to 'headers_factory' and
made it optional.

- Added 'headers_factory' to upload/download functions.

- Allow 201 "CREATED" status code in upload_file.