Drop py3.7 from CI; cut run to 5mins

.github/workflows/ci.yml

@@ -24,13 +24,13 @@ jobs:
 
   testing:
     name: '${{ matrix.os }} Python ${{ matrix.python }} - ${{ matrix.spawn_backend }}'
-    timeout-minutes: 10
+    timeout-minutes: 5
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest]
-        python: ['3.7', '3.8', '3.9']
+        python: ['3.8', '3.9']
         spawn_backend: ['trio', 'mp']
     steps:
       - name: Checkout