Run GitHub Actions on legacy Python 2.7

ahupp#326 (comment)

Author: cclauss

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ['ubuntu-latest']
-        python-version: ['3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
+        python-version: ['2.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
         include:
           - os: macos-latest
             python-version: '3.13'