CI: update Python version matrix

Old Python versions do not support the modern type annotation
syntax. It's time to update the test matrix.

We'll drop 3.8 and 3.9, adding 3.12 and 3.14 instead.

.github/workflows/unit-tests.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.10", "3.11", "3.12", "3.14"]
         architecture: ["x64"]
 
     steps: