Ubuntu test fixes

Author: mrvisscher

.github/workflows/testing.yaml

@@ -15,7 +15,8 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-13, macos-latest]
         py-version: ["3.10", "3.11", "3.12"]
-
+    env:
+      DISPLAY: ':99.0'
     steps:
       - uses: actions/checkout@v4