Add LICENSE, fix PEP 639 classifier warning, enhance --version output, add dev dependencies

- Create MIT LICENSE file
- Remove deprecated License classifier, add license-files to pyproject.toml
- Add __license__ constant to __init__.py
- Show full name, copyright, and license in rp --version
- Add ruff and mypy as dev dependencies

src/rp/__init__.py

@@ -2,3 +2,4 @@
 
 __version__ = "0.2.0"
 __author__ = "Yunxiao Xu"
+__license__ = "MIT"