updated python to 3.9 in the CI

Author: jacobwilliams

.github/workflows/CI.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         gcc_v: [7,8,9,10] # Version of GFortran we want to use.
-        python-version: [3.7]
+        python-version: [3.9]
     env:
       FC: gfortran-${{matrix.gcc_v}}
       GCC_V: ${{matrix.gcc_v}}