[ci] test: add 3.9 python version

Author: cbentejac

.github/workflows/continuous-integration.yml

@@ -24,7 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ 3.11 ]
+        python-version: [ 3.11, 3.9 ]
 
     steps:
     - uses: actions/checkout@v3
@@ -61,7 +61,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        python-version: [ 3.11 ]
+        python-version: [ 3.11, 3.9 ]
 
     steps:
       - uses: actions/checkout@v3