Add back Python 3.8 on Ubuntu 24.04

This was recently added to the config so it should now work:
actions/python-versions@dd24c2e

.github/workflows/main.yml

@@ -43,12 +43,6 @@ jobs:
       matrix:
         os: [ 'ubuntu-24.04', 'ubuntu-22.04' ]
         python: [ '3.12', '3.11', '3.10', '3.9','3.8' ]
-        exclude:
-            # GitHub Actions doesn't provide Python 3.8 for Ubuntu 24.04;
-            # all of the available Python versions are listed here:
-            # https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
-          - os: 'ubuntu-24.04'
-            python: '3.8'
 
     runs-on: ${{ matrix.os }}
     name: Python ${{ matrix.python }} on ${{ matrix.os }}