Add Ubuntu 24.04 and macOS 13

This required removing Python 3.7 which was EOL'd in 2023, as it's no longer
available in newer OS versions:

> Error: The version '3.7' with architecture 'x64' was not found for Ubuntu
> 24.04. The list of all available versions can be found here:
> https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Python 3.7 EOL as of 2023-06-27:

* https://devguide.python.org/versions/
* https://peps.python.org/pep-0537/

.github/workflows/main.yml

@@ -36,8 +36,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ 'ubuntu-22.04', 'macos-12' ]
-        python: [ '3.7', '3.8', '3.9', '3.10', '3.11' ]
+        os: [ 'ubuntu-24.04', 'ubuntu-22.04', 'macos-13', 'macos-12' ]
+        python: [ '3.8', '3.9', '3.10', '3.11' ]
     name: Python ${{ matrix.python }} (${{ matrix.os }})
     steps:
       - name: Checkout repo