bump ubuntu release on github actions to 20.04

.github/workflows/pypi_release.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   build-n-publish:
     name: Build and publish Python 🐍 distributions 📦 to PyPI
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@master
     - name: Set up Python 3.10