tools: github actions: ubuntu-18.04 runners are no longer available

- https://docs.github.com/en/actions/using-jobs/choosing-the-runner-for-a-job

.github/workflows/ci.yml

@@ -2,7 +2,7 @@ name: python-shaarli-client CI
 on: [push, pull_request]
 jobs:
   tox:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9']