Bump version 0.14.1 → 0.15.0

Breaking changes

• Exception classes have been moved to httpx_oauth.exceptions module.
• Exception classes now have a required message property.
• GetAccessTokenError, RefreshTokenError and RevokeTokenError accept an optional response property.

New features and improvements

• Add support for client_secret_basic authentication method.
• Improved error handling: network errors and JSON decode errors are now properly handled.
• Code is now entirely documented with docstrings.