Simplug

- feat: allow plugin.register to be used as a decorator

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.4.2...0.4.3

- deps: add setuptools for test
- chore: improve documentation for loading plugins from setuptools entry_points
- chore: improve error message for multiple implementations when single result is expected

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.4.1...0.4.2

- feat: allow plugins to not exist with `-` for plugins_context

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.4.0...0.4.1

- depr: deprecate `plugins_only_context()` and `plugins_but_context()`, use `plugins_context()` instead
- ci: use latest actions
- style: fix mypy issues

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.3.2...0.4.0

- ⬆️ Bump diot to 0.2.1

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.3.1...0.3.2

- ⬆️ Bump diot to 0.2

**Full Changelog**: https://github.com/pwwang/simplug/compare/0.3.0...0.3.1