[Iamrodos]

Python 3.10 reached end of life on 2026-10-01 and no longer receives security fixes. This PR requires Python 3.11 or higher, following #442, which dropped 3.8 and 3.9 after they reached end of life.

This PR makes the following changes:

• setup.py sets python_requires=">=3.11" and drops the 3.10 classifier.
• The CI test and lint matrices drop 3.10.
• The README's requirements list says Python 3.11 or higher.

No code changes are needed: nothing in the code depends on 3.10, and nothing can be simplified with 3.11.

Compatibility

• On Python 3.10, pip install github-backup keeps installing the last release that supports 3.10. Pinning a newer version fails with a clear Requires-Python error.
• Anyone on a system whose Python is 3.10 can stay on the last release that supports it, or install a newer Python with uv or pyenv.
• The Docker image uses Python 3.12 and is unaffected.

Testing

• Installing on Python 3.10 is refused with Requires-Python >=3.11. On 3.11 it installs and runs.
• pytest passes on 3.11, and rst-lint README.rst passes.