fix: Add compatibility module for StrEnum and assert_never (#12)

[YazdanRa]

Note

This pull request is a fix to the #12

This pull request introduces a new compatibility module to support Python versions earlier than 3.11, ensuring that features like StrEnum and assert_never are available across all supported environments. The main changes involve refactoring imports throughout the codebase to use these compatibility shims, improving cross-version compatibility and maintainability.

Python compatibility improvements:

• Added a new module chatkit/_compat.py that provides backports for StrEnum and assert_never for Python versions < 3.11.
• Updated imports in chatkit/errors.py, chatkit/agents.py, chatkit/server.py, and tests/helpers/mock_widget.py to use StrEnum and assert_never from the new _compat module instead of directly from the standard library.
• Removed direct imports of assert_never from the typing module in files where it is now provided by _compat.

These changes ensure that the codebase remains functional and consistent across both older and newer Python versions.