ci: replace pytest with unittest

Using pytest, we run into access violation errors on Windows.
They disappear when we use the unittest module.

.github/workflows/tests.yml

@@ -60,7 +60,7 @@ jobs:
           function execute() { echo -e "\033[0;34m$*\033[0m"; "$@"; }
           
           execute source venv/bin/activate
-          execute xvfb-run python -m pytest
+          execute xvfb-run python -m unittest
 
   test-windows:
     runs-on: windows-latest