CI: Drop Python 3.8, add 3.14 and 3.15

As we use the fedora-python/tox-github-action@main,
it was updated to Fedora 42 which no longer has Python 3.8.

https://fedoraproject.org/wiki/Changes/RetirePython3.8

Author: hroncok

.github/workflows/main.yml

@@ -22,12 +22,13 @@ jobs:
     strategy:
       matrix:
         tox_env:
-          - py38
           - py39
           - py310
           - py311
           - py312
           - py313
+          - py314
+          - py315
           - mypy
 
     # Use GitHub's Linux Docker host