Update tests.yml

Author: tk-king

.github/workflows/tests.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   build-n-publish:
     name: Tests
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
     - name: Set up Python 3.9