Fix CI issue for python 3.7

[pengyunie]

GitHub workflow's ubuntu-latest now points to 24.04, where Python 3.7 is not available any more.
To fix it, I changed the CI config to let Python 3.7 use ubuntu-22.04. Other versions (3.8, 3.9. 3.10) stay the same.

It has been a few years since we published pytest-inline. I'm planning to do the following upgrades to CI / build config recently, let me know if there is any objections:

1. Add CI tests for newer Python versions (3.11 -- 3.14)
2. Upgrade the build and test toolchain to use uv, which should replace conda and tox
3. We should consider dropping support for older Python versions (3.7, 3.8, 3.9, which are all end of life by now) at some point

[Copilot]

Pull Request Overview

This PR fixes a CI failure caused by Python 3.7 no longer being available on ubuntu-latest (which now points to Ubuntu 24.04). The solution pins Python 3.7 to Ubuntu 22.04 while keeping other Python versions on the latest Ubuntu release.

• Updated the GitHub Actions workflow to use a dynamic runs-on configuration based on the Python version
• Added a matrix configuration to specify Ubuntu 22.04 for Python 3.7 tests
• Maintained support for Python 3.8, 3.9, and 3.10 on ubuntu-latest

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[Copilot]

[nitpick] "The matrix configuration will create unintended duplicate job runs. With the current structure, Python versions 3.8, 3.9, and 3.10 will each run on ubuntu-latest (as intended), but then Python 3.7 will be added as an additional job. However, because both python-version and runs-on are defined as matrix dimensions, this creates a cartesian product: 3 Python versions × 1 OS = 3 jobs, plus 1 job from the include. The configuration is correct as-is. To make it clearer, you could restructure to avoid having runs-on as a matrix variable and instead only use include entries, like this:

matrix:
fail-fast: false
include:
```suggestion
        include:
          - python-version: '3.8'
            runs-on: ubuntu-latest
          - python-version: '3.9'
            runs-on: ubuntu-latest
          - python-version: '3.10'
            runs-on: ubuntu-latest