Clean-python

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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.

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- Manage.update now automatically retries if a Conflict is raised.

- AlreadyExists is not a subclass of Conflict anymore.

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- Fixed verification of client credentials tokens.

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- Adapted RequestQuery.filters() to deal with list query parameters.

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- Skip health check access logs.

- Fix access logging of correlation id.

- Workaround celery issues with message headers: use the body (kwargs) instead.