Update GitHub Actions to use Ubuntu 20.04 and install Python 3.5

[Copilot]

• Update build-test.yml workflow to use ubuntu-20.04 runner (line 15)
• Update github-pages.yml workflow to use ubuntu-20.04 runner (lines 18 and 48 - build and deploy jobs)
• Verify workflows are syntactically valid
• Ensure Python 3.5 compatibility with ubuntu-20.04

Summary of Changes:
Updated all GitHub Actions workflows in .github/workflows/ to use ubuntu-20.04 instead of ubuntu-latest. This ensures Python 3.5 can be installed successfully, as newer Ubuntu versions (22.04+) no longer support Python 3.5.

Files Modified:

• .github/workflows/build-test.yml: Changed runner from ubuntu-latest to ubuntu-20.04
• .github/workflows/github-pages.yml: Changed both build and deploy job runners from ubuntu-latest to ubuntu-20.04

Verification:

• All workflow files validated as correct YAML syntax
• Python 3.5.10 confirmed available for ubuntu-20.04 via actions/python-versions manifest
• Code review passed with no issues
• Security scan completed with no vulnerabilities detected

Original prompt

@copilot on the branch pelican of https://github.com/pythonclub/pythonclub.github.io repository, update all existant actions on .github/workflows to use the ubuntu 20.04 and to ensure that python3.5 is installable .
Create a pull-request
https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

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[luzfcb]

@copilot try again.

[Copilot]

@copilot try again.

Updated workflows to use ubuntu-20.04. Changes committed in 48ee71d.

[luzfcb]

@copilot you create a pull request on @luzfcb/pythonclub.github.io instead of @pythonclub/pythonclub.github.io

[Copilot]

@copilot you create a pull request on @luzfcb/pythonclub.github.io instead of @pythonclub/pythonclub.github.io

I'm working in the fork (luzfcb/pythonclub.github.io) and can only push changes here. The changes are ready in branch copilot/update-workflows-ubuntu-20-04 (commit 48ee71d). You'll need to create the PR from your fork to pythonclub/pythonclub.github.io manually, or I need direct access to push to the upstream repository.