Remove Python 3.8 from CI workflow matrix

Author: Pierre-Louis Barbarant

.github/workflows/unit_tests.yml

@@ -15,8 +15,6 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
-            version_python: "3.8"
           - os: ubuntu-latest
             version_python: "3.9"
           - os: ubuntu-latest