v0.9.0

Added

• New support method, is_value_defined to check if a config field value is defined by the user,
  through either a loaded config file or the API.
• New support method, reset_value to reset a configuration value back to the default.
• New Config method, Config._set_default_value, to set a field default value. This was added so
  that __setdefault__ methods wouldn't need to access the _data dictionary directly.
• New options to the DictField that control key and value validation.

Internal API Changes

• Switched to GitHub Actions.
• Switched to ruff, pyright, and black for linting and formatting.
• Switched to poetry for dependency management.