[deps] Add support for Python 3.10, 3.11, 3.12 & 3.13

[pandafy]

• Drop support for Python 3.6, 3.7 and 3.8

[nemesifier]

Expanding this issue into the NetEngine modernization work needed to support Python 3.10, 3.11, 3.12, and 3.13.

Planned scope:

• Replace the unschedulable ubuntu-20.04 CI runner with ubuntu-24.04, and align Actions versions and pip caching with NetJSONConfig.
• Add python_requires >=3.10, Python 3.10 to 3.13 classifiers, and documented supported versions.
• Add Dependabot version updates for pip and GitHub Actions. Dependabot security updates are already enabled.
• Upgrade netaddr, migrate the legacy pysnmp 4 cmdgen backend and its tests to a supported pysnmp 7 API, and preserve SNMP get and walk behavior.
• Replace the old QA stack with the NetJSONConfig-compatible tooling: nose2[coverage_plugin]>=0.16.0, Coveralls, Sphinx, and OpenWISP Utils 1.3 QA checks. Replace the old link-validator server block with sphinx-build -W.
• Add NetJSONConfig-equivalent OpenWISP Utils workflows that NetEngine lacks: backport, CI-failure handling, and version-branch replication. Existing assignment, stale-PR, reopen, PR-link, and changelog bots remain in place.

[openwisp-companion]

This issue has been automatically assigned to @nemesifier who opened PR #75 to address it. 🎯