[CI] Dropped support for Python 3.9

[atif09]

Python 3.9 is no longer supported and is causing CI failures, this aligns with other OpenWISP repositories that have already dropped Python 3.9 support.

References #49

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request raises the minimum supported Python version from 3.9 to 3.10 across the project. Changes remove Python 3.9 from the GitHub Actions CI matrix, update README.rst to require Python >=3.10, and modify setup.py to reflect the new minimum Python requirement and to adjust classifiers (removing 3.9 and adding 3.13). No other workflow logic or public API changes are present.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description directly relates to the changeset, explaining that Python 3.9 is no longer supported and is causing CI failures, with the change aligning with other OpenWISP repositories.
Title check	✅ Passed	The title directly summarizes the main change: dropping Python 3.9 support from CI, which is reflected across all modified files (CI workflow, README, and setup.py).

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