feat: Enforce Python 3.8+ requirement and add multi-version CI testing

- Add python_requires=">=3.8" to setup.py to enforce minimum version at install time
- Update README to explicitly document Python 3.8+ requirement
- Add CI matrix to test lint/build on Python 3.8-3.14 (7 versions)
- Aligns with actual usage patterns (~99% of downloads on Python 3.8+)
- Prevents future PRs from inadvertently using incompatible syntax

This change protects users by preventing installation on unsupported Python
versions and ensures contributors can see version requirements clearly.

setup.py

@@ -50,5 +50,6 @@ setup(
     long_description=open_file("README.rst").read(),
     long_description_content_type="text/x-rst",
     install_requires=open_file("requirements.txt").readlines(),
+    python_requires=">=3.8",
     zip_safe=True,
 )