reduce CI matrix to only test oldest and newest supported Python/Ruby versions

Author: slonopotamus

.github/workflows/ci.yml

@@ -8,8 +8,8 @@ jobs:
   build:
     strategy:
       matrix:
-        ruby: [ 'jruby-9.2', '2.3', '2.4', '2.5', '2.6', '2.7', '3.0' ]
-        python: [ '3.5', '3.6', '3.7', '3.8', '3.9' ]
+        ruby: [ 'jruby-9.2', '2.3', '3.0' ]
+        python: [ '3.5', '3.9' ]
         platform: [ ubuntu-latest, macos-latest, windows-latest ]
     runs-on: ${{ matrix.platform }}
     steps: