Depreciate python 3.7 and 3.8 for the linter, and use 3.9 as in the docker image

Author: JulienPeloton

.github/workflows/linter.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8]
+        python-version: [3.9]
 
     steps:
     - uses: actions/checkout@v2