Carto

- BatchSQLClient.cancel is now resilient to 'Cannot set status from done to cancelled' errors
- Fixed a race condition in BatchSQLClient tests
- Fixed URL relative path join problem in `is_valid_api_key`

- Added a `CopySQLClient` for efficient streaming of data to and from CARTO (87)
- Fixed CI tests
- Added a request header `User-Agent: carto-python-sdk/x.y.z` where `x.y.z` is the current version of the library.
- Added a `client=cps-x.y.z` parameter to requests, where `x.y.z` is the current version of the library.

- Updated release instructions in CONTRIBUTING.md

- Added AuthAPIClient class for future Auth API usage and API key validation (69)

- Added PROTOCOL and HOST to test configuration allowing local testing

- Fix on examples management of environment variables (70)
- Fix onpremises tests (70)