Dropped support for Python 3.9

[Srinath0916]

Checklist

[x] I have read the OpenWISP Contributing Guidelines.
[x] I have manually tested the changes proposed in this pull request.
[ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
[x] I have updated the documentation.
Reference to Existing Issue
This addresses the Python 3.9 CI failures affecting multiple PRs. The maintainers have indicated that PRs for dropping Python 3.9 support are welcome.

Description of Changes
This PR drops support for Python 3.9, which is reaching end-of-life in October 2025 and causing CI failures due to dependency compatibility issues.

Changes made:

Removed Python 3.9 from the CI workflow matrix in .github/workflows/ci.yml
Removed Python 3.9 exclusion rules from the CI matrix
Updated documentation to require Python >= 3.10 in docs/developer/installation.rst
Added changelog entry in CHANGES.rst
This follows the same approach as openwisp-monitoring (commit fa80622) and will help unblock PRs that are currently failing due to Python 3.9 dependency issues.

Screenshot
Not applicable - this is a configuration/documentation change.

[Srinath0916]

Can someone review it and approve the workflow ?

[coveralls]

coverage: 98.607%. remained the same
when pulling 9abf8cd on Srinath0916:drop-python-3.9
into faec934 on openwisp:master.

[pandafy]

As @DragnEmperor suggested in the development chat, we also need to update the Dockerfile to use Python 3,10

[pandafy]

We prepare the changelog during the release, so we can skip this in this commit.

[Srinath0916]

Thanks @pandafy , I've addressed both points,

Updated Dockerfile to use Python 3.10
Removed the changelog entry from CHANGES.rst

The changes are ready for review. Could you also approve the workflow runs when you get a chance?

[pandafy]

Thanks for the PR @Srinath0916!

[Srinath0916]

Thank you @pandafy , Happy to help unblock the CI. Looking forward to contributing more to OpenWISP!🚀