ci: move to ubuntu-24-04 runner

Need to adjust Python versions as a result

Author: daviddrysdale

.github/workflows/ci.yml

@@ -6,10 +6,10 @@ permissions:
 jobs:
   build:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", 3.11, 3.12, pypy2.7, pypy3.9]
+        python-version: [3.8, 3.9, "3.10", 3.11, 3.12, 3.13, pypy2.7, pypy3.9]
 
     steps:
     - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3